The advice given so far is "real advice". And if you don't consider that to be, then how bout this? Play smarter. But just in case that's too vague, here's what I mean...
Position better - As any type of carry at all, after the laning phase you should be with your team/support/tank(s), and position yourself in a way so that your teammates can peel for you, or at the very least, drop him after he drops you. If he's in your match, and you're playing a carry, ALWAYS assume that he's right around the corner ready to jump on you from stealth - unless you can actually see otherwise on the map.
Ward - As a carry against a stealther JG buy 'multiple' pink wards, one for lane (bush) and one for dropping randomly if he's nearby or if a skirmish/teamfight breaks out. And as anything other than a carry, also buy multiple pink wards. Don't rely on your support to ward/pink for you, it's just as much YOUR job to cover your own back and that of your team's, as it is anyone else's.
If it's a stealther JG, consider putting your yellow wards out further than you usually would if it was a normal JG. Position wards based on their ganking potential.
Play Safe - When there's a stealther, or anyone who can gank with relative ease... play safe in lane. Freeze as much as possible, unless you are aware of where Rengar is. Communication helps this, and so does WARDING. If there are no wards available, and you don't know where he is/are a vulnerable champ to him, back off often. Just as well, let your teammates know Rengar was spotted wherever, a caution ping is usually enough, even if it's your own lane. Or a ping in the direction he went after. This gives other lanes the ability to react accordingly - pushing, being aggressive, backing off, etc.
Also, don't face check bushes, period. And ward them from blind spots, whenever possible (if you're going to).
Champ Picks - Other than that, play any champ at all that has even remote tankiness and/or CC... Seriously. Most that fall under the category of tanks, bruisers, and "juggernauts" will murder rengar 1v1, he's an assassin first and foremost. My guess if you think Rengar is this unstoppable, then chances are you don't play anything but carries. And even as a carry, do all else above and be amazed at how much harder it is for Rengar to blow you up.
Even some extra mobility can help.
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zhoyanas on an adc is actually not bad lmao especially jhin
Why would you ever buy Zhonya's on an Adc instead of GA?...
GA actually benefits the Adc in that #1 it gives both types of resists and more armor, #2 forces Rengar to use his cooldowns to get you down the first time, rather than just waiting 2.5 seconds, on top of you, to kill you in .2 - instead it's 4 second stasis you don't even have to activate, and gives your teammates time to down him. #3 it costs way less and doesn't waste stats. I would think this would be reason enough alone. #4 It forces him and his teammates to choose to go after you just to pop the GA buff, and put himself in danger to do so - or get this, go after someone else. Imagine that.
Will never in my life figure out why some carries, are so opposed to buying GAs... especially when an Adc vs any assassin at all.