Not a preexisting monster, but your own.

I'd be a living pile of garbage, because I already am one.

A sad blob

Either a ghost that lives in Snowdin, or some kind of bird that acts as a mail woman/messenger between the different towns.

... idfk. just wanna be a ghost thats freakin ballin.

Bird monster

Skeleton.

Ghost

Wolfian

Idk but I KNOW THAT I WON'T BE A BOSS 😭

Some sort of feline-like animal that radiates good vibes

I would be a pair of shoes holding a computer

I would probably be homeless and some type of rodent.

myself already