Horse rears continuously, leading to server crash on logout

While riding a horse, I began to encounter strange behavior (having to shift multiple times to dismount, more frequent and extended rearing, etc). Eventually my horse got stuck in the rearing animation for several seconds, and my player would not dismount after many shift presses. Finally, I logged out. When I did so, the server crashed.

Upon restarting the server and loggging in the horse was gone.

The server console displayed the following messages: https://gist.githubusercontent.com/nicksloan/ada0a9dd1e4711b2043a95cbc17ecfde/raw/fcdecac8bc97b5d8ea797333a5e02159e2fbba43/gistfile1.txt

Here is the crash report: https://gist.githubusercontent.com/nicksloan/34e870d7499e139b1a7dd28316d2b6f0/raw/0d238bda1ca35356fd3019bb7aed922860ecbe4d/gistfile1.txt