Frenzied Ally (3.5e Feat)
From D&D Wiki
Frenzied Ally [General]
When your animal companion believes itself outnumbered, outmaneuvered, or cornered it fights with unmatched aggression and brutality.
Prerequisite: You must have an animal companion and the Vicious Ally feat.
Benefit: When your animal companion is flanked by one or more hostile creatures, or is within 15 feet of 4 or more enemies, or cannot leave its square without provoking an attack of opportunity, it enters a state of furious rage. This state of rage grants it a bonus +4 Strength or Dexterity as well as +4 Constitution. Additionally its Will and Reflex saves are increased by +2, and the animal companion gains immunity to fear effects. The animal companion remains in a frenzied state equal to 3 + its newly improved Constitution modifier. Your animal companion cannot willingly end its frenzied state nor can you cause it (or otherwise command it) to exit this state, but it is not Fatigued at the end of its rage. The benefits of this feat stack with other rage effects that may be applied to the animal companion.