It's a Shadow Mastiff or something with the Shadow Creature template from Lords of Madness. It could also be a Shadurakul from the Fiend Folio, listed with the other Canomorphs, which is basically a lycanthrope combination of Shadow Mastiff and Human.

Animate Objects at will sounds like maybe a Fiend of Possession, a prestige class also in the Fiend Folio, which makes a lot of sense. It can possess and animate anything from a statue to a puddle of water to your buddy's corpse, and whatever it's possessing gains the stats of an animated object. The FoP is unaffected by any damage dealt to anything it possesses, and there are only a few very specific spells which will even affect it while it is possessing something, because otherwise the spell targets the creature or object it possesses. Magic Circle Against Evil, Protection from Evil, or a nearby Unicorn (via Summon Nature's Ally) would be extremely useful if this is the case.