I have a Paint that is MUCH too smart. He has a VERY good idea of the level of the rider on top of him and will act accordingly. Absolute beginners (especially little ones) he is a doll and will go around perfect. When they have more experience he begins to test more. He is also one of those horses where if you don't ask right, he ain't going to do it! (Pointing and kicking go no where)
Now that I've owned him many years now, he has really amazed me with how observant he is. He has picked up on things I like and don't, and 'reads my mind' on a regular basis.
I've been riding my friends horse as she has been unable to due to an injury, and her horse is smart in learning things very quickly! In one ride we had a 360* difference, he just wants to learn!
Every time I go to the barn another horse surprises me in how smart they are, they are amazing animals :)
I've ridden quite a few horses and have encountered horses who don't have the aptitude to learn as quickly as others. My mare will learn something if I know how to teach it to her in one session and its there forever. If I don't know what I'm doing trying to teach her something new that is also new to me, it takes a little longer, but we still get it right eventually.
My horse is very smart, when he decides he wants to be. When I teach him something, he'll pretend not to understand, then, if I don't teach it to him for a couple days, when I go back and try again, he does it perfectly. Being an almost two year old, he does do some dumb things, but that is only because he's still learning. It helps that I've spent every day working with him since he was 2 months old, and know him like the back of my hand, but he's the best horse I could have asked for.