Just FYI, the way I make ears is just that: the way I make them. But let's be schooled a bit anyway, because I like anatomy too much.

Dogs have something called cutaneous marginal pouch (see //not my image//). It's believed that it may explain why dogs/cats/bats···· hear ultrasound.
I love drawing it, I just find it super cute- but when I draw cartoon I just exaggerate the shape without detailing.
See Rentham's ears: http://www.furaffinity.net/view/16824467/
And another example in my last pic, where they're kinda more detailed (?): http://www.furaffinity.net/view/17020238/

I'm saying this because I've seen more than one person drawing Ren's ears like if they were cut, and nope. Why does Ren have them if he's a robot? Because yes.
Anyway his ears are just long, bald and with that droopy shape. Do whatever you want if you respect that, idc XD

I know this journal is like extremely useless, but I felt the urge to explain it. :v