Object not rotating right https://gyazo.com/1bcb0ae93258d5d19efc42b5a008ec59

I tried parenting an object to a bone but when I rotate it it doesn't rotate as I expect, it is not supposed to leave a little gap between the horse body and the saddle, why does it rotate like that?
If you want the saddle to be "stuck" to the back of the character, you could parent it to 3 vertices of the face you want the saddle to be stuck to.


Select the saddle (RMB)
Add the horse to the selection (SHIFT + RMB)
Toggle Edit Mode
Select 3 vertices
Make Vertex Parent (CTRL + P)