I played Minecraft with my friend in Realms today.

At that time, I hit my horse by mistake, then my dogs that have been already tamed killed it!
In the local worlds, tamed dogs never kill any tamed horses, however, their behavior is different in Realms.

I think it is a bug.

We tested whether it is an only once bug. After reentering the world, my friend hit his horse for a test, then his dogs killed it!

My device is an Android, and so is my friend's.

Please check whether it is a bug.