Ironskinned animal companions have strong connections to the elemental plane of earth.
Ironskinned Animal Companion (Ex)
A character able to select an animal companion can choose an animal companion with the ironskinned template.
Section 15: Copyright Notice – Adventurer’s Handbook: Genius Guide Volume 1
Adventurer’s Handbook: Genius Guide Volume 1. Copyright 2010, Super Genius Games. Authors: Owen K.C. Stephens and Stan!