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.