I use to have a companion addon which would show in the tooltip if a mob dropped a companion .eg. The Baradin Fox would say in the tooltip drops Fox kit.
I use GupPet for this - it marks mobs who drop companion pets and mounts, as well as vendors that sell them, and also works as a random-companion and random-mount addon.
It also has an in-game list (with models) of all known companion pets and mounts, and it'll tell you where to get them.