I'm about to start a PbP in my homebrew setting, in which one of my players will be playing a Paladin of Hades (who in my world is LN, to stave off impending alignment questions). Said player is looking for a replacement idea for her Divine Bond mount besides a horse. Given her patron and the character's love of all things fuzzy, a Cerberus Pup was the obvious answer.
Since Divine Bond already says "as a Druid's animal companion of the Paladin's level", the easiest way to manage the base creature is to start with the stats of a Dog or Wolf companion (probably Wolf, since it starts at Medium and moves up to Large; I can't imagine a Cerberus Pup as anything smaller than Medium, personally) and add another two Bite attacks. Then as she levels, give it weakened versions of the actual Cerberus's special abilities (fear howl, petrifying gaze, poison breath) instead of the normal progression of the Paladin mount (Celestial template, SR) or some of the Druid companion abilities.
Note the Paladin will not actually be riding the Pup much, given her skirmishy, Spring Attack-based fighting style; it'll be more a companion, flanker, and fighting partner.
You are free to make a house rule of course.
Yes, that's kind of the intent, considering that's the forum it's in and this critter doesn't exist in the game =)
I wouldn't be granting it that second bite, obviously, since it would have it from the start and then some, I agree it would be overkill for it to get yet another, plus a fourth would invite questions as to why one head can bite twice but the other two can't. Plus, it has three natural attacks anyway, so that level would give it Multiattack instead.
One possibility I was considering was having its bites damage be sized one size smaller than it actually is - 1d4 instead of 1d6 while it's Medium, 1d6 instead of 1d8 when it becomes Large - to reflect that it is, still, a puppy.
You might get more ideas here,
Might want to have her spend a feat (say around 7th level... leadership...)
It's not that she acquires a cohort, it's that the pup becomes a cohort... and has additional powers/abilities. (I think this is one way a halfling paladin can gain a Blink Dog mount)
So: starts off as a 3 headed wolf (+2 bite attacks, probably a bonus to grapple..., and int starts off around 6-7...)
It then becomes large.
Lesser versions of the classic powers at first
Instead of fear howl, demoralizing howl (intimidate)
Petrifying gaze could be replaced by paralyzing or slow gaze (hold monster or slow), nauseating breath instead of poison breath... perhaps some fire resistance as well? (the game gives it out like candy...)