Hrmm...I have a Ravager with the highest rank of Gore/Bite, Great Stamina, Avoidance, and some in Natural Armor. I was going to ask what works best for PvP, but it seems like Scorpids are the way to go.
I also have a freshly-tamed Trachela sitting in my stable. Is it worth keeping as a pet designed to annoy the crap out of people in BGs?
I've seen birds used in BG's and they are annoying, but they are more of a grinding/soloing pet. Basically for BG's it doesn't really matter what pet you have, as gear is more important.
As for abilities, I always train the max ranks of Great Stamina, Growl (its free), Avoidance, Cobra Reflexes. Then I will train a DPS ability (claw if they have it, otherwise bite, but not both) and a pet type specific ability (Gore for Ravagers, screech for birds, lightning breath for windserpents, scorpid poison for scorpids, etc), and since I'm impatient I will pick up dash/dive. Alot of hunters will put points into resistances (shadow and frost for pvp, instance dependant for raiders) but I dislike training resistances as with careful management it's not really required, and usually go for armor to use up the remaining points.