It depends on the character, anon! Along what lines were you thinking?

If it's a living, breathing creature that's capable of interacting with humans and independent and intelligent thought, then there's no reason it shouldn't be allowed. However, if it's larger than a normal human, it ought to be sized down! So, basically as long as it's not say, Godzilla or am actual Titan from Attack on Titan, then it should be fine. Humanoid-type characters like Hellboy, Abe Sapien, or Garrus from Mass Effect would all be fine.

Machines like Transformers would have to be humanized and sized down in some way to make them fit with the setting a bit easier however.

I hope that helps!