Itumaak (and I assume the other animals) seem to have the equivalent of 0 athletic skills and gets fatigued extremely easily. My lvl 9 party has a minimum of 3 athletics and after a few fights none of my party is tired while Itumaak suffers critical fatigue.
Critical fatigue = -15 all defense, -30 accuracy, 50% max endurance
It makes him useless in combat after a few fights and I suspect most people don't know this is happening because the only indicator is buried at the bottom of the hunters stat sheet.
I even tried to cheat and give him 10 athletic skill and it didn't help. My guess is that animal_companions don't use skills and they don't ever increase their fatigue resistance.