Why is Rengar so freaking OP?

Over the past week there has been an increase these rengar top laners that insta-kill pretty much everyone now with just 3 items. Even full armor tanks just gone in like two seconds and there's really no way to counter it (if there is please post it). I even played a game were nobody on the enemy team had any kills except rengar. we shut them down in every other lane (except top) but there was nothing we were able to do because he would just jump out every 60 seconds and kill 2 people on the team before u could even react. Pinks don't do anything to combat the damage or even give you much of reaction time since he can jump from farther range then the pink covers. By the time anyone can even click to exhaust him its already to late. Every fight was just and instant 3 v 5. Pretty much all they do build a last whipser, Ravenous Hydra, and a youmuus and its GG.

If anyone can enlighten us on how to counter a mid/late game rengar plz post.