You have an undead animal companion.
Benefit(s): Your animal companion’s type changes to undead, but its Hit Dice, base attack bonus, saving throws, skills, and tricks are retained from the base creature. The creature loses its Constitution score and its Charisma score becomes 12. If your companion is destroyed, your new companion is undead as well, using these same modifications.
Pathfinder Roleplaying Game Monster Codex © 2014, Paizo Inc.; Authors: Dennis Baker, Jesse Benner, Logan Bonner, Jason Bulmahn, Ross Byers, John Compton, Robert N. Emerson, Jonathan H. Keith, Dale C. McCoy, Jr., Mark Moreland, Tom Phillips, Stephen Radney-MacFarland, Sean K Reynolds, Thomas M. Reid, Patrick Renie, Mark Seifter, Tork Shaw, Neil Spicer, Owen K.C. Stephens, and Russ Taylor.