Why not Pete? He's a pretty big guy?

The Rat Ogre!

There's a lot of options in Tokyo AfterSchool Summoners, Housamo in short.

Gouryou, Volos, Rave, Cusith, Gyobu, Makara, Typhon, Chernobog, Ashigara, Jambavan, Shino, Moritaka, Tsathoggua, and many others.

Can't pay but I can offer you options.

Always do ladies, ladies are the bomb

Malifecent?
Charizard?
Blastoise?
Birdo/Yoshi?
Epona?

That big yellow yoshi thing from galaxy I think, he counts as big and kinda plump. Can't think of his name though, didn't play the game. Uhm.. lugia's sorta.. big in the back door in some art. *shrug*

What the heck, I'll throw out the idea of King K. Rool, might as well, yeah? Sadly I don't have a paypal nor any money to spare so it saddens me I can't have mah cario wrigglin' up a croc rump.

There is Sajin from Bleach. Big fox guy. As for pokemon surprised nobody mentioned Snorlax or Slaking.

Asgore comes to mind.

Asgore is a classic boss dad. I also recommend Wolf O'Donnel and K Rool because theyre hot and also topical right now

Pumbaa.. Foghorn! Those two are so very rare

Mister Pete indeed

If you plan on drawing them from the front, a few female characters will certainly increase your consumer base on account of all the people who don't much care for AV but do enjoy UB and will appreciate an angle that leaves it up to the imagination which one of the two holes into which they may be spelunking.

Probably wouldn't pay for such a thing myself (mostly because my finances are tight right now), but here are a few by which I would definitely be tempted (and will be very surprised if you have no other takers for if offered):

Ben Clawhauser from Zootopia
Chunky from The Croods
Kevin from Up
Marahute from The Rescuers Down Under
Maximus from Tangled
Toothless from How To Train Your Dragon

An Oozaru from the Dragonball franchise!

Can't forget King Dedede. He's sportin' a monstah to clobbah that they-ah Kirby! It's what he does best in sodomy~!

An incineroar that's let himself go? Sounds kinda silly, but I like it.

i could make a baneposting meme here

i'm just gonna say it: Guilmon/charizard