I am making a dungeon but I forgot the name of the monster that can possess a structure, and control things like doors and even rearrange passageways. 

If I remember correctly, to kill it you need to destroy the wall that it is currently in.

I am 80% sure that this exists. Does anyone know the name of this monster?

I do not know anything like that.  Here are the ones I think of that sound similar to what you describe.:

Eidolon

Hoard Mimic

Trapper

this is the only example i can think of
https://myheroacademia.fandom.com/wiki/Mimicry

fraz-urb'llu(the prince of lies and weilder of the staff of power) can cause walls to turn into doors and vice versa while in his (fraz-urb'llu(the prince of lies and weilder of the staff of power)'s) lair (zoragmelok, located in hollow's heart, lair of fraz-urb'llu(the prince of lies and weilder of the staff of power)