After working for weeks my Petri finally reached level 60 and became third form. PM me ingame if you'd like to see it. Here's the basic info (I left out base stats because I'm pretty sure they're on the wiki...):
Name:
Pengman
Modes:
Full Loot Mode
Half Loot Half Attack
25% pickup and 75% Attack
Nothing
Description:
"Pengman spout a large 3rd middle horn in the middle of their head allowing them to penetrate enemies causing major damage. Their personality remains fairly unchanged from Petruskis."
I hope you enjoy this new information! (Also, even though the description is truthful about the third horn, the third form is quite cute.)