Is he like Rammus where he's a mutant and completely unique?
His lore seems to imply that there are others of his kind, however it doesn't state if his race even has a name.
Since there is no bestiary for League I thought I'd check around here to see people's thoughts.
Also should I start a listing of all creatures in Valoran, with the communities help?

Bump

There was an explosion about this topic back when Rengar was released. I hate to break it to you, but most people don't really care any more.

It was an interesting thing to think about, but in the context of the League, not very relevant. Think about how many faaar stranger beings in the league there are. A tangible nightmare, and incarnation of flame, gods of ice, extradimensional beings, etc. All of these make a lion-man seem not so irregular any more.

If you have any cool insights about him though, please do post them. I'm sure the community would be receptive to theories, but I don't think they really want to spend much time thinking about Rengar's origins right now.

****ing *******s that don't care about lore.
short awnser,he is a charr

I would say a leonin. It'd be nice if Riot actually said something about it though. I like to know what the thing is that I'm playing.

Actually, he's a lion. A white lion (Yes White Lions exist)

Apparently he and Nidalee are the same species (through means of magical osmosis)? Both hailing from the Kumungu Jungle?.

So, cat people. or Leonin.

He's probably from Kumungu Jungles where there is a tribe of beings similar to him. Been doing research, and we have to consider the magic in the Rune Wars affect much of the natural world in Valoran. Think like Nidalee's family of cougar but empowered by magic.