A group of PCs want to take control of a dead enemy Cavalier's mount.
I cannot for the life of me find rules on this. Surely the mount would not have all of the same abilities which it possessed while a companion of the Cavalier.
I just can't find anything which explains how to handle this situation.
Intriguing question scallywag. From a player standpoint I can see why they'd want to get a really cool mount. Am presuming this is a horse. Also presuming we're giving or looking for an option for the party to do so. So my two cents, just for fun...
From Cavalier I think some relevant text is: A cavalier's bond with his mount is strong, with the pair learning to anticipate each other's moods and moves. Should a cavalier's mount die, the cavalier may find another mount to serve him after 1 week of mourning.
Think a good starting point is the mount reacting in a similar manner. It too mourns for 1 week for the missing part of itself it lost, the enemy rider. From a role play angle I'd presume the players must use handle animal or another class feature to begin to build a relationship with animal as it overcomes it's loss and maybe skittishness if it was treated poorly, etc.
From a crunch standpoint I imagine a lot of folks might disagree, but one option is to keep HD and HP as is. Then regress the feats back to beginning two + the lt armor proficiency, plus skills of a regular riding horse. If you decide to let them unlock skills, feats, abilities, or tricks the horse is capable of they would need to either retrain the animal or figure out what the horse knows and what commands it responds to. Could be any number of words in a foreign language or signals from the rider that they'd might need to spend a good deal of time to figure out. Perhaps even researching the training habits of the enemy as a small side adventure.
I'd say that it would be fine to take if the player who takes the mount also has to take 2-3 feats to accommodate for the mount. Consider that you can get a fully leveled animal companion (and some minor skill boosts) for 3 feats. As long as the players are willing to commit to it and care for the mount, I don't see why they can't have a really cool horse. Until the players have collectively invested 3 or so feats into owning the mount, they shouldn't get the full benefit- the mount should probably get spooked (and run) in combat, since they aren't with their proper owner. Thus, the players just have a really cool war spoils horse until they have spent the feats to actually own it. As for how to stat it out before then, if the players seem attached enough to invest feats into the mount, have it retain its full HP and saves (so it doesn't die randomly), but use the skills of a regular horse, and have it run from or be more or less useless (but not actively harmful) in combat. If they kick it into a tunnel to check for traps, treat it as a regular horse in terms of HP.