Did you use a saddle?
I just found someone else's report and a mod's response:
[Mod] Skylinerw added a comment - 17/Oct/16 11:53 AM
As of 1.11, mobs will take damage when occupying the same space. The threshold is specified by the maxEntityCramming gamerule (defaulting to 24).
You can raise this number via commands, right?
(Update) Yes you can, the command is "/gamerule maxEntityCramming"
Put a number after 'maxEntityCramming'.
Also for fun: if you want to touch a mob and kill it, set it to 1.
I am glad to see that this issue has been noticed and tracked. Yet it has been a long time since the bug was reported. I do not know what kind of cooperation this website has with Mojang, but does anyone know what kind of progress has been made towards fixing this?
No, there was still one horse there.
One of them went away, so I'm gonna have to run all the way back to get another one.
Take note that this glitch applies to blocks and dropped items too.
This applies to dogs too.