So in our last session of Dungeon World the Ranger's animal companion came under direct attack by a PC. The pet in question is a huge 'eagle' called Trace. We're sure it's actually a disguised dragon whelp or young Roc, but the Ranger just grins whenever we ask her how she got her… unique… pet.
The PC in question, the Thief, had just joined the campaign and decided he was a foreigner who had arrived in the last few days, thus having no chance of any personal history with the current group. Approaching our camp at night he missed on a Defy Danger roll to remain unseen by us and was subsequently attacked and mildly wounded by the patrolling 'eagle', which of course he promptly stabbed in retaliation. At the time we ruled along the lines of the Man's Best Friend move (which the Ranger doesn't actually have) and put the companion out of action until it had rested for a while.
If the companion's attacker had been a NPC/Monster this would have been more easily handled as the GM could just narrate what happened, including the effects on the companion and what it would take to lift them. However in this case the Thief, since he didn't know what this terrifying flapping monstrosity was, was making a dedicated bid to murder it, triggering a move which inflicts HP damage (Hack and Slash) on something lacking defined HP.
Looking through the rulebook afterwards (out of curiosity) I was surprised to find a distinct lack of guidance and advice on animal companions and the Man's Best Friend move is the only place where harm to the companion is mentioned at all.
This binary active/not-active state doesn't really sit well with me outside of that move's scope, as it leaves me zero guidance on how life-threatening a given amount of HP damage would be to an animal companion.
How can I get an idea of how severe the fictional harm to an animal companion is?
Aside: I noted that mechanically the animal companion mechanics seem superficially similar to Apocalypse World's Driver mechanics (Command is very similar to No @#$% Driver) and AW provides guidelines in its rules for how vehicles, NPCs and gangs generally react to harm but I can't figure out how this guidance might be converted.