Hey guys, so I have a strange question seeing as I'm not well versed in the monsters of D&D. Also, apologies for any formatting errors.

So my DM is wanting to run an in-universe one-shot next weekend and she asked me to help out. Basically, she wants me to play as a monster in disguise; when the party gets to the end of the dungeon and is fresh out of resources, I'll turn back into my monstrous form and beat the crap out of them. The biggest problem I'm having is that I don't know which creatures can change their appearance, or which creatures would make sense to give the ability to change appearance.

The party right now is built out of 5 level 3 characters not including me (I'll be there boss they'll be fighting). Can you wizards help me find a shapeshifting creature and balance a hard/deadly boss fight around 5 level 3s that are fairly out of resources?