Handle Animal: Can use this skill to advance a creature +1 HD, as a check of DC 20+the creature's current HD, taking 1 week. Neither the creature’s HD nor CR can exceed 5 less than the number of ranks you have in Handle Animal.
... the idea is that the resulting creature will NEVER go beyond the HD value of even the Ranger's Animal Companion. Of course, Animal Companions also get things like share spells and evasion, so there's no threat of this trumping any animal companion or mount provided by a class feature.
Nevertheless, for characters such as, say, a Roughrider Fighter Archetype, this is a fantastic option.
Whaddya think? Good houserule?