What campaigns feature you playing as a monster?

[Zephandrypus]

The other races and characters are too normal, monsters are more fun. So far all the ones I know of are:
Ooze Awakening

[Telchin]

It's probably shameless to advertise my own creation, but by your description I think you might like it. My UMC is called Inky's Quest (currently on 1.12 server) and is about sea monsters (particularly cuttlefish). Just like you I was bored with most mainline factions being just humans by another name (elves, orcs, etc.), so I decided to make a monster focused campaign. I chose the Cuttlefish because I figured they would have very different gameplay from humanoids (as they're fast in deep water, but slow on flat land). The story isn't particularly deep (pun intended ) and the difficulty is very low, but I tried for the campaign to be humorous and for its scenarios to be something more unusuall than just "kill all enemy leaders". (You can post your opinion in the thread linked in my signature.)

[watbesh]

I'm not sure this can count as a "monster" you mention, but Danse Macabre may qualify. The first motivation of that campaign was "We want to create a campaign where an undead leads, and it should be a low-tier corpse, not any sort of necromancer".

kamikaze (the lead creator) and his team, Wesnoth Undead Research Institute, seem to be dead(!) for years, but James_the_Invisible has raised the campaign from its grave so it should be playable. Its monstrous difficulty probably stays.