I've seen some avatars with a deer face. Is that an item or...? c:

It's a zapper skin, which will be available next month through the V2 Zapper for the Morostide holiday. You zap your avatar for 50 Wizard Tokens and you get a random skin from a this list. New ones might be added this year, but we don't know for sure yet!

Ah, thank you :3

Is it possible to get normal skin back afterwards?

Yep! On that page I linked you to, there is a little line of text under each category that says "can be cured using...", which tells you the specific cure you'd need for the type of skin you got. Each cure can be made through the Asmodeus Genetech Lab.