Vermin Companion( Eberron Campaign Setting, p. 62)
Instead of an animal companion, you have a vermin creature as a companion.
Child of Winter (ECS) , ability to acquire a new animal companion, Druid level 3rd, minimum level requirement (see below), nongood alignment,
When choosing an animal companion, you may choose a vermin instead. Even though your companion is a vermin, you can cast spells on it that normally target animals as though it were an animal. Your companion gains an Intelligence score of 1 but does not gain any skills or feats. Apply the indicated adjustment to the character's level (in parentheses) to determine the companion's special powers (see Alternative Animal Companions, page 36 of the Player's Handbook). Vermin Companions. 3rd Level or Higher (Level -0). Giant ant, worker; Giant fire beetle; Giant ant, soldier; Monstrous centipede, Medium. Giant bee; Monstrous scorpion, Small. Giant bombardier beetle; Monstrous spider, Small. 4th Level or Higher (Level -3). Giant praying mantis;;Giant wasp. 7th Level or Higher (Level -6). Giant stag beetle. A vermin companion has all the normal abilities of a typical creature of its kind, as well as the characteristics of an animal companion determined by your druid level.