The Anubis would be one of the most effective enemies at slaying Epona, except that it evidently doesn't want to. Only the gem on an Anubis's head is vulnerable to Epona's attacks, but because Anubises float so high, Epona can only hit one when it is emerging from the ground. Even then, this hit only stuns it for a moment and does not deal damage, so Epona cannot defeat an Anubis. In addition, an Anubis could easily roast Epona with fireballs while floating too high for her to even stun it. However, Anubises refuse to attack Epona. Instead, they simply float close to her and watch carefully. You may know that Anubises counterattack some of Link's attacks with a fireball, but they will never counterattack Epona's attacks, even if she successfully hits one. Considering how easily they could defeat Epona, yet refuse to ever attack her under any circumstances, Anubises must really like horses.