You should be able to 'g'ive an alraunia antidote to a poisoned pet/companion to cure them. There could be a message like "The *foo* eagerly consumes the herbs. The *foo* seems cured."

In fact I'm not sure they even recover from poison at all naturally. You *can* cure them with thrown potions of cure poison, but those are kind of limited. The herb would make using companions for a longer time a more viable playstyle. It also could be given to poisoned neutral monsters for a lawful boost.

This probably shouldn't work on some monster types like constructs.