The Argent Tournament mounts have a spell called "Refresh mount". Allows you to heal your mount while out of combat for 50,000 HP, with a 1 minute cool down. However if you spam-click the spell you can get off 4-5 heals at once before the cool down goes into effect. Probably not supposed to work like this.

I was able to heal for 250K. Right now the mount I tested this on only have 50,000 HP so it's not a big deal as it's basically a 200K over heal, however I am not sure if different mounts have different health pools or not. Figured I would report it anyways just in case it could be used as an exploit some how.