After reading the new lore that's probably going to introduce the new champion, i kept thinking Rengar might be one of them. It makes sense, doesnt it?
Just mentioning it in hopes a Rioter from world building might have something to comment.
Riot's lore team is the slowest on the multiverse it might take a few centuries before we find out the answer to your question.
Oh look an image from the Vastaya page with a Vastayan that looks awfully like Rengar (with the feline head, colors and all).
I'm not Riot member, but I think I have a relatively solid answer to your question.
In the voice lines for Xayah, she says something about rengar being a Vastayan.
https://www.youtube.com/watch?v=INYpl_Rb5Us skip to 10:17 to hear the line.
