Source: Dragon #326
You gain a monstrous creature as an animal companion.
Prerequisite: Ability to gain an animal companion, Savage Empathy.
Benefit: Add celestial owl, darkmantle, fiendish raven, giant fire beetle, and stirge to the list of creatures from which you may select your animal companion. As with normal animal companions, you may gain more powerful companions as you progress in level. However, you cannot simply summon these more powerful creatures as druids and rangers normally do. Rather, to gain such a monstrous companion you must first encounter such a creature and change its attitude to friendly or helpful. Changing the creature's attitude is most likely an effect of using either the Savage Empathy feat or the spell charm monster. If your monstrous companion's attitude ever changes to indifferent or worse, such as when a charm monster spell ends, the monster ceases to be your companion and may either leave or attack, as its nature dictates. The following lists serve the same purpose as those presented in the Alternative Animal Companions section of The Druid's Animal Companion sidebar on page 36 of the Player's Handbook.
4th level or Higher (level 3)
7th level or Higher (level 6)
10th level or Higher (level 9)
13th level or Higher (level 12)
16th level or Higher (level 15)