Theory about Pierrot's Monter Form

We can assume that each of the characters have animalistic traits or even forms, In this case it's assumed Doctor is a sort of Bird monster, given his Plague Doctor mask.

So then if they do take after animals, then would everyone else be?

I suppose Harlequin has a more clear idea given his tentacles and forked tongue, but I'll talk about Pierrot.

It's hard to come up with an idea of what this beautiful man could be for there aren't any hints, HOWEVER, there was someone who pointed something out about this AMA question:

I don't know how I haven't seen this before, but the fact that he states "under the water", implies he's a creature that swims, because why would a land creature willingly be underwater to look at a sunset. 

So we get he's probably an aquatic creature, but what now.

So you see...

In the game when we take a look at Columbina's past, we get to see the silhouettes of all their true forms:

With his bulky silhouette, he must be a big animal, especially with how he has spikes, and this could be a stretch but I like to think he has scales which would make him a reptilian type of animal.

This leads me to the conclusion that he could be a crocodile type of creature due to his robust figure.

There's little evidence of this but this was just a thought

(LOL as I wrote this, Neko hinted to us that this screenshot wouldn't help us for he states, "The silhouettes from Columbina’s era won’t help you much, because they were really very young and... mistreated by the circus, so some "parts" might be missing or covered by worn-out clothes in future updates, you’ll have more clues from the present to go off of!" comment on thoseofthatilk Tumbr post Let’s play, guess Pierrot’s monster design before Neko releases it.) The analysis this person did os so good (ToT) pls check it out if you can, it has some great theories and notes.