I dredged up the image below from an old post on Dragonsfoot. A previous owner customized the Wandering Monsters chart in one poster's Holmes Basic D&D rules. Click the pic to get a larger version.
I'm guessing that "Hydra 3" on the third level chart is meant to indicate a 3-headed version of that beastie.
Imagine for a moment the possibility that out there somewhere is some poor bastard whose first encounter in D&D was with a hostile dungeon chicken.