AP regnar is actually a real thing. The other two are more gimmicks imo, but rengar does work very well as an AP assassin. He essentially does the same thing as AD rengar, but he just does it differently. Once you get DFG, you just ult by someone, dfg them, then hit W twice. At the point they are dead or very wounded.
He is also about impossible to push out of lane too. It's really hard to trade with someone that has an AoE damage aura thing that scales so well. He works fine as AD too, but AP rengar does work. Though the one in the screencap didn't really build that well. You want to get as much AP as quick as possible to beef up your dfg. You think lichbane would make sense, but it is something you would get later, if at all.