I chose undecided, and my humble opinion on the matter is this:
If you have not gone to school for human massage, then certification is a good option. Most of these "certification" programs have been started by massage therapists - like you and I, who also have a burning passion for the welfare of animals - and have branched out to include marketing to massage therapists (who may be interested in changing or adding to their practice). Unfortunately, there is something amiss, a big something (speaking from direct experience).
Much of the program focus is on massage theory & a variety of techniques, based on the human body. Well, frankly, we've all been there and done that. There are very few programs out there with the prerequisite of being a human massage therapist to enroll.
In the future, as the profession of animal massage continues to grow, certification from some of these programs may hold legal standing. This is the only reason I chose undecided. A massage therapist (with knowledge of animal behavior) can learn as much, if not more, on their own...there is a lot to be said for being self-taught. There are many classes available through colleges, and there are many animal shelters and rescue organizations to donate time and gain hands-on experience.
I carry coverage with ABMP for animal work. In order to receive coverage, I needed to provide proof of certification. Check out another great thread regarding animal massage ethics & law: http://www.massageplanet.com/forum/viewtopic.php?t=3028
I am currently enrolled in a professional equine acupressure practitioner program (boy, that's a mouthful!). I'm looking at probably 1.5 - 2 yrs to complete it, on a part-time basis. Part of this program is online (maybe half) the other half I'll have to travel throughout the country to get in the hands-on portion. They are based out of Colorado, maybe I'll be going there...whoohoo! I chose this course because of their excellent reputation, and also because it is not a repeat of an education that I've already had. Certification is not cheap!
In regard to equine massage, I don't work with race horses but there are therapists whose appointment books are filled up with horses only...yes, I believe they would be elite race or show.
Hope my little schpeel (sp?) was helpful to you. Good luck in your animal adventures!