Horses easily are suffocated. I made a big stable and it was complete so I started spawning horses. It was a 4X6 stall with walls with two block space in the middle, three blocks lengthwise of space to be in. I spawned the horses wit the spawn egg. I spawned about 200 and 2/3s of them suffocated! A lot of them were foals and I really got frustrated and had to mute the game because of all the death sounds. Foals often walked into the walls and suffocated themselves without any reason. This sometimes happened to adult horses!