How to counter Rengar
Let us take a look on how to beat Rengar. We will look at champions who are strong against Rengar and who are weak against Rengar. Use win rate, gold difference and creep score difference at minute 10 to find the best 
Jungle champion who counters Rengar. You can already win in champion select with the right counters for Rengar. If you want to know how to counter Rengar then you need to take into account the best picks vs Rengar and the worst picks vs Rengar. This Rengar counter data is based on matches played by Pros on patch 14.12 and previous ones. It is important to see whether Pros counter picked Rengar or whether they played as Rengar.
Champions used by Pros to counter pick Rengar
Looking at the matches from Pros (see table Pros counter picked Rengar) we can see that they picked 
Lee Sin most often to counter pick Rengar. The matchup Lee Sin vs Rengar occured 20 times. Rengar has 12 wins and 8 losses in this matchup. Learn how to counter pick Lee Sin to get another perspective on the data.
Lane outcome for Rengar
When looking at Rengar counters you will notice that win rates alone are not conclusional. Therefore we provide an overview (see table Lane Outcome) where you can see the lane outcome of Pros who played Rengar. The lane outcome of Rengar consists of three outcomes: Win, Loss or Draw. Let's take a look of Rengar lane and match outcome for specific matchups. Considering the lane outcome and the match outcome of a specific matchup, might enable you to find the right counter for Rengar.
Conclusion
Analyze the available data and learn how to beat Rengar. To master Rengar take also a look on Rengar builds and runes for Rengar. Good luck, Summoner!