Baldur's Gate : The Sword Coast Chronicles : NWN2 Persistent World

In this particular case you need to report this player with screenshots provided, although I doubt that DMs would punish him. Sorry to hear about what happened.

I would expect pack horse to be treated as animal companion or a familiar - meaning "extension of their master" and stuff (even though it is a bit silly sounding). Because they aren't an easy to replace summon.

But you might need rule clarification or official ruling for this one.

The person who attacked your horse was a hacker pretending to be another player. He killed multiple horses and several npc's before being kicked by Dm Wisp. If you see anything similar tell a dm as soon as possible or contact a player who is also a dm.