It takes a lot for a horse to trip and fall. They have four on the floor, which makes them pretty stable. (Yes I meant the pun.) I've seen horses slip on the mud, just yesterday in fact, but to trip and fall the way we humans do? No. 

I tried all sorts of sketches, that failed as long as two feet were touching the ground. Suffice to say that if your horse is stumbling around, you should look into it.
If your horse looks anything like this cartoon, then take a picture, send it in and then call your vet.