My horse can't be leashed
TL;DR; My horse was previously leashed but got glitched.
So I've been thinking about this for a while, and since my horse despawned, here is my theory why he's broken.
The anarchy world is multiplayer, meaning my friend and I were the ones playing around with him. It seems that my horse was leashed somewhere else I can't find. When I typed in /tp @e @s to get my horse, it might've had an impact of glitching the horse's behaviour.
It can explain why he can't get on boats nor get leashed, because he is leashed somewhere but got glitched over.
In that case, I regret using that command, but I would still use it for other reasons.