So Nirah is the only manticore character we know of in the game, but he doesn't really give any indications about what his past might be or where he came from. Surely he can't be the only manticore in existence, so he has to come from somewhere.

There's one decor item that I think may point to an answer; the Mefistofelis. The item's description is "Will this demonic Felis lure your lions into the world of sin, like it has done with Manticores?" What does it mean "like it has done with manticores"? Could manticores be in the hell faction?

The name of the Mefistofelis item is a play on "Mefistopheles", a demon which appears in German folklore, most prominently in the legend of Faust. In that story, he corrupts Faust by offering him great knowledge, power, and pleasure at the cost of his soul. Faust takes the deal, but at the end, he is dragged to hell. With such a clear reference to this legend, there's no way they aren't related. Maybe the manticores are either trapped in hell or serve in hell, and Nirah somehow managed to get away from that. Nirah also says that demons dislike him because he's not "evil enough" and angels dislike him because he isn't pure. Maybe it's less so Nirah himself isn't pure, but manticores in general aren't considered so, perhaps because they're servants of hell.

Nirah himself also seems aggressive toward demons. In one of his dialogue lines, he yells at the player character thinking they are a demon: "NO, I don't want to join your cult! Oh! Sorry I thought you were one of those demon messengers again." Even while Nirah also seems to dislike heaven, he seems to downright hate hell. He talks about angels being creepy, but when he talks about demons he seems spiteful, talking about how demons are enslaving "all the animals you've befriended in the past month and making them all slave-warriors to bend to their will and reign chaos onto the earth." So yeah, he doesn't seem to fond of them

TL;DR version: The manticores were tricked into becoming slave-servants to hell and Nirah managed to escape somehow and make it to earth to warn of the upcoming heaven vs hell battle.