The best theory available for the loch ness monster is that it was probably a worm

Congal Cláiringnech ("the cripple"),[1] son of Rudraige, was, according to medieval Irish legend and historical tradition, a king of Ulster and High King of Ireland. He was the brother of Bresal Bó-Díbad, the former High King, who had been killed by Lugaid Luaigne.

While Lugaid Luaigne was High King of Ireland, Congall shared the kingship of Ulster with Fergus mac Léti, ruling the northern half of the province while Fergus ruled the southern half. The Ulstermen objected to being ruled by two kings, and both submitted to the judgement of the High King at Tara as to which should rule the province. Lugaid decided to give the kingship to Fergus, who his daughter Findabair had fallen in love with, and compensate Congal with land, status and gold, but Congal refused and declared war. He was supported by some of the Ulster noblemen, including Fergus mac Róich and Bricriu, as well as allies from the other Irish provinces and from Scotland. Fergus mac Léti also called upon his allies, including Fachtna Fáthach from Ulster, Cet mac Mágach from Connacht and Mesgegra from Leinster, and there were great losses on both sides.

 The chronology of Geoffrey Keating's Foras Feasa ar Éirinn dates his reign to 135–120 BC,[4] 

Ness (Irish: Neasa, Nessa; Old Irish: Neas, Ness), also called Nessa, is a princess of the Ulaid and the mother of Conchobar mac Nessa in the Ulster Cycle of Irish mythology. Her father is Eochaid Sálbuide, king of the Ulaid.

Ness sits on a flagstone by the river Conchobar, and the following morning gives birth. The baby falls into the river, but Cathbad lifts him out, names him Conchobar after the river, and brings him up as his own son.[3]

After the Táin, Conchobar falls ill and doesn't eat or sleep. The Ulaid ask Cathbad to find out what's wrong with their king. Conchobar tells Cathbad that he is ill because the other four provinces of Ireland have made war against him with impunity. Although he was victorious against Ailill and Medb, neither of them was killed in the battle, and he still lost his bull. He wants to make war against Connacht, but it is now winter, so Cathbad advises him to wait until summer when his men and horses will be fresh and energetic, and in the meantime, call on all his foreign allies to bring reinforcements. He sends word to Conall Cernach, who is raising tribute in the Scottish islands, and he raises a great fleet of the Ulaid's allies in Scandinavia and the Faroe Islands and brings them home to Ulster.

 local stone carvings by the Pict depict a mysterious beast with flippers. The first written account appears in a biography of St. Columba from 565 AD. 

This is how big the biggest earthworms in UK are:

These not only don't go in water much at all (though they both can be in the soil nearby), there is no way that you can suggest that is what the legend is based on, since the origins is further back than this:

Medieval Irish historical tradition held that Ireland had been ruled by an Ard Rí or High King since ancient times, and compilations like the 11th-century Lebor Gabála Érenn

Many of these kings are considered to be legendary. Dynastic affiliations are based on the genealogies of historical dynasties who claimed them as an ancestor.

Taking the native dating as broadly accurate, another theory has emerged. The Roman historian Tacitus mentions that Agricola, while governor of Roman Britain (AD 78–84), entertained an exiled Irish prince, thinking to use him as a pretext for a possible conquest of Ireland.[12] Neither Agricola nor his successors ever conquered Ireland, but in recent years archaeology has challenged the belief that the Romans never set foot on the island. Roman and Romano-British artefacts have been found primarily in Leinster, notably a fortified site on the promontory of Drumanagh, fifteen miles north of Dublin, and burials on the nearby island of Lambay, both close to where Túathal is supposed to have landed, and other sites associated with Túathal such as Tara and Clogher. However, whether this is evidence of trade, diplomacy or military activity is a matter of controversy. It is possible that the Romans may have given support to Túathal, or someone like him, to regain his throne in the interests of having a friendly neighbour who could restrain Irish raiding.[5][13] The 2nd-century Roman poet Juvenal, who may have served in Britain under Agricola, wrote that "arms had been taken beyond the shores of Ireland",[14] and the coincidence of dates is striking.

The Three Collas (Modern Irish: Trí Cholla) were, according to medieval Irish legend and historical tradition, the fourth-century sons of Eochaid Doimlén, son of Cairbre Lifechair. Their names were: Cairell Colla Uais; Muiredach Colla Fo Chrí (also spelt Colla Dá Crich, or Fochrich); and Áed Colla Menn. Colla Uais ruled as High King of Ireland for four years.[1] Recent DNA analysis confirms the history of the Three Collas in fourth-century Ireland, but questions their descent from Eochaid Doimlén and Cairbre Lifechair.

According to the Sanas Cormaic,[3] Crimthand Mór mac Fidaig built a great fortress in Cornwall known as Dind Traduí or Dinn Tradui (Dun Tredui/e, fortress of the three ramparts).[4] There appears to be little doubt that it existed,[5]

 local stone carvings by the Pict depict a mysterious beast with flippers. The first written account appears in a biography of St. Columba from 565 AD. 

The first claimed sighting of the Loch Ness Monster was in the River Ness in AD 565, when Saint Columba is said to have banished a "water monster" back into the river after it tried to attack one of his disciples who was swimming across the river.[17]

According to Adomnán, when Columba visited King Bridei I of Pictland at his house on the River Ness, he met a wizard named Broichan who had an Irish slave-girl that he refused to release even though Columba pleaded with him. Columba went out of Bridei's house and picked up a white pebble from the river. He said that the pebble would be used to heal many sick people in Pictland, and that Broichan was suffering for his sins at that very moment. After he had finished speaking, two messengers came to tell them that Broichan had a seizure and they wanted Columba to help them. Columba gave them the stone and said to dip it in water to give to Broichan, if he agreed to release the slave-girl. He agreed to do so, and the stone was put in water and it floated on it; the wizard drank from the water and was healed. This stone was kept by King Bridei in the royal treasury for the rest of his life, and anyone who came there for healing would be given water with the stone floating in it, and they would be healed.[18]

So, while it's crystal clear what my opponent is trying to do here, it's frankly insulting to all proud Scots to suggest they nabbed their folklore legend from the Irish, since Princess Ness was alive after the legend of the Loch Ness monster had already begun to surface, as evidenced by records of sightings.

As she and Cathbad set out to visit Fachtna, Ness goes into labour. Cathbad tells her if she can manage not to give birth until the following day, her son will be a great king and have everlasting fame, for he will be born on the same day as Jesus Christ. Ness sits on a flagstone by the river Conchobar, and the following morning gives birth. The baby falls into the river, but Cathbad lifts him out, names him Conchobar after the river, and brings him up as his own son.[3]

Since it dates so far back, it's possible that some kind of dinosaur remnant that has since gone extinct was genuinely in the waters, the most likely relative (or creature itself) was this:

The hydronym Ness is of Pictish origin.[3] The name may be derived from *Nessa, the name of a river goddess.[3] 

I am not fucking okay with being mocked and accused of writing anything with pedophilic undertones. This is fucking bullshit.

According to Adomnán, when Columba visited King Bridei I of Pictland at his house on the River Ness, he met a wizard named Broichan who had an Irish slave-girl 

Also, that legend in Ireland was actually potentially 'stolen' from Iceland. 

According to both Landnámabók and Íslendingabók, monks known as the Papar lived in Iceland before Scandinavian settlers arrived, possibly members of a Hiberno-Scottish mission. Recent archaeological excavations have revealed the ruins of a cabin in Hafnir on the Reykjanes peninsula. Carbon dating indicates that it was abandoned sometime between 770 and 880.

As with the Scottish lake Loch Ness, a cryptid serpent, called Lagarfljótsormurinn by locals, is believed by some to live in the depths of Lagarfljót.

So far Pro has given 0% proof of the loch Ness monster. Instead, Pro has begun to commit extreme ad hominem, strawman and gish gallop.

The Picts were first noticed in AD 297, when a Roman writer spoke of the “Picts and Irish [Scots] attacking” Hadrian’s Wall.

Loch Ness monster, byname Nessie, large marine creature believed by some people to inhabit Loch Ness, Scotland. However, much of the alleged evidence supporting its existence has been discredited, and it is widely thought that the monster is a myth.

The Scots didn't know much about the Irish as tbe cultures developed. Pro may be trying to say it's a gaelic celtic folklore tale. In this case, I demand proof. Not proof that the named Nessa was around in Ireland, proof that it's a worm.

 He also concludes that the story of Saint Columba may have been impacted by earlier Irish myths about the Caoránach and an Oilliphéist.[24]

Pro has proven absolutely nothing other than that the Irish has a legend about worms and that some famous people in Ireland had the name Ness or Nessa.

The head and neck of Loch Ness monster move exactly like a swan or goose do (especially swan but from sime angles goose). The rest of the body cannit possibly be a tiny worm, that is outrageous to claim.

These not only don't go in water much at all (though they both can be in the soil nearby), there is no way that you can suggest that is what the legend is based on, since the origins is further back than this:

 What I'm referring to is a painting/drawing that strongly implies it was a legend spoken of in Scotland as far bacj as before 500BC.

Reports of a monster inhabiting Loch Ness date back to ancient times. Notably, local stone carvings by the Pict depict a mysterious beast with flippers. The first written account appears in a biography of St. Columba from 565 AD.

So, while it's crystal clear what my opponent is trying to do here, it's frankly insulting to all proud Scots to suggest they nabbed their folklore legend from the Irish

The Scottish people (Scots: Scots Fowk; Scottish Gaelic: Albannaich, Old English: Scottas) or Scots are a nation and ethnic group native to Scotland. Historically, they emerged from an amalgamation of two Celtic-speaking peoples, the Picts and Gaels, who founded the Kingdom of Scotland (or Alba) in the 9th century

Also, that legend in Ireland was actually potentially 'stolen' from Iceland. 

This is a 1-day-per-round debate where my opponent posts rapid reponses, so life got in the way and I forfeit 1/5 Rounds. I'm sorry but I genuinely couldn't make the deadline for IRL reasons.

I refuse to respond to Pro's rebuttals at this point and here is why; Pro hasn't proven anything, he is demanding I prove it.

The size of British worms make it absolutely unfeasible that this is what the Loch Ness monster would be, unless explicitly stated to be it (which it isn't).

The head and neck of the Loch Ness monster on any photo pretty much equals a swan or goose silhouette.

While geese are more common in Canada than Britain, it doesn't matter all that much, my second link is an English goose and my primary theory is a swan late at night.

However, I am much more sure that long before cameras existed andthis 'photo' which may be a giant eel was around, the actual sightings were swans.

The only other explanation is that it's a remnant of the dinosaur type I linked to in Round 1, I didn't explicitly say that's my direct theory.

Pro's Round 1 is a series of random references to 'Ness' or 'Nessa'. It has absolutely no link between one thing to the next and his overall case contradicts itself since why on Earth would a Princess of the Royal Family of Ireland be named after a hideous worm?

This entire thing makes very little sense, the most Pro has proven is that Ireland has a tale about a worm.