The Bug:
The hitboxes of horses, donkeys, and mules are not adjusted when they rear.
Steps to Reproduce:
Enable entity hitboxes by pressing "F3 + B".
Summon a horse, obtain a bow, and hold it in your hand.
Right-click on the horse to make it rear.
Take note as to whether or not the hitboxes of horses, donkeys, and mules are not adjusted when they rear.
Observed Behavior:
The hitboxes are not adjusted when they rear.
Expected Behavior:
The hitboxes would be adjusted when they rear.
Related issues
Attachments
Comments

Can confirm that this is an issue in 1.16.5 and 21w14a. I'm happy to take ownership of this ticket, since the reporter's account has been marked as inactive.
This issue isn't exclusive to horses and can also be seen with donkeys and mules. Since I'm the reporter of this ticket, I've made the appropriate changes to reflect this.

Affects 1.20.1 Release Candidate 1