When using the Horse Spawn Egg in Creative, and placed it on a flat vertical wall, the Horse suffocated. I expected it to just fall down, but it stayed in place and started suffocating. I tried this with other mob eggs and they fell down, but the horse stayed and suffocated until it died. It suffocates if you place it in a verticle block atlest 1 block high off the ground. All you have to do if stack 3 blocks high, then place it on the second or the highest block and it will die.