Is there a multiple tile entity/creature in the game

Like is there a gargantuan abomination that I could fight in the game?

Yes and no; while vehicle span multiple tiles, creatures are always one tile, no matter their actual size (similar to Dwarf Fortress).

Yes. Even if they are technically just one tile big (some tilesets actually render them bigger than one tile, but they still take up only one tile as far as the game’s concerned), there are quite a few very big creatures…
One that probably fits your needs very well would be the shoggoth, which’s description starts with “A gargantuan protoplasmic blob […]”.
Other than that, there are - amongst others - jabberwock, Exodii quadruped, and the Combat Mech.
Mods might add even more colossal creatures.

They might not fit through a window, but they can still follow you through an 1 tile wide alley.

Are there non vanilla multi tile entity. Like creatures from mods.

Was that a question or a statement? If it was a question: No.
To my knowledge, the game currently can only process one tile sized creatures.

Mods like the DinoMod add extremly large creatures (with a volume of 1000 Liter and 1000 kg, the max before the game makes their corpses vanish), but they still are only one tile in size.

That said; in collapsed towers there are these weird flesh walls (and fungal towers keep growing / spawning in mutliple fungal tendrils), which could be said to belong to a larger creature, but even then you still fight them individually.

If the code was in place for multitile monsters you can bet we’d move out chonkier vanillla mobs over. Candidates would be armored centipedes, the biggest insects, dissoluted devourers, all of Dinomod…

The latest improvement to autodrive lay out so good basis for mutiltile monsters. But no one is working on using it for that currently

How likely will multi tile creatures be added?

Based on the current development direction, it can’t be ruled out completely that one day there will be creatures that span multiple tiles.
But just as Fris0urman stated, no one’s currently working on that, so it’s unlikely to happen “soon”.

Great. Can’t wait to fight a 20 by 20 tile sized collosal boss monster.

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There are some plenty big monsters though.

That’s great and all but multi tile creatures would be more fun to fight. You could choose specific parts of the creature to attack. It could make for a good boss fight. .

If you want it in the near term you’ll either have to develop it yourself or persuade someone else to do it.