From PRD: "Animal companions do not gain additional attacks using their natural weapons for a high base attack bonus."
At level 9 animal companions get the multiattack class feature if not replaced by any animal archetype.
Animal companions with only 1 or 2 natural attacks (such as Wolf or Pony) will get a second attack at their BAB-5. But that's just a coincidence. It is not related to high BAB. And they would never get a third or forth attack due to an even higher BAB.
Animal companions with 3 or more natural attacks get -5 on all their secondary attacks until they get multiattack which reduces the penalty to -2.
Animal companions with 3 or more natural attacks that are all primary attacks (i.e. they don't have a secondary attack), get no benefit from multiattack.
I'm opting to use ASPECT:CombatBonus to handle this. There is no method to add a second attack at -5 to a Natural Attack today, and I'm not going to set up a Fake Second attack, since Weapon Focus and all that would be a pain to hack on.
Until the Fomula Parser is up and able to handle this request, I believe this is the best option for now.