The new Ionian story, pretty much confirm what Rengar is: A Vastaya

So if you havent read it yet, go here. TLDR: The Vastaya are a hybrid/chimeric race of varying half human, half animal(s) that originate from Ionia. Some are more animal than others. is a Vastaya. They have magical qualities, all of them, some are capable of shapeshifting between more humanoid forms and others between more animal forms. The upcoming champion(s) rumored "The Harpy and the Phoenix" are Vasyatas.

This pretty much gives us a resonable idea of what Rengar is: A Vastaya who was probably lost as a child, before his hunter father in his lore adopted and raised him. This would also explain why he is capable of semingly turning invisible without technology, he comes from a magical race who have varying powers. To further exemplify the probability that Rengar is a Vestaya, is this image in the Universe page (click here).

On the left we see a white lion-man vastaya almost identical to rengar, just we fewer dreadlocks (he/she has 2). Mystery solved.