Sounds like it used to be a Beast in 3.0, a creature type that doesn't exist anymore. Yeah, the HD would become normal Animal HD, as with any Druid Animal Companion.
As for skills, an animal gets skill points for its HD an Int modifier as if its Animal HD were its class, that is to say, 2+Int (this will most certaily be only the minimum 1 per HD). Class skills are undefined. It seems safe to assume that the skills the animal originally has ranks in are class skills for its hit dice, for anything else, ask the DM.