Hunter pets are still suffering from a long-standing scaling issue.
The Bug: pets start out at ~6k hp base. After several seconds (sometimes as many as five, which is a long time in PvP combat) the pet scales up to the hunter's Stamina.
Reproducing It: This bug can be seen when dismounting from a flying mount directly into combat, or summoning a new pet, or reviving a pet using Revive Pet or Heart of the Phoenix (which revives the pet at the location it perished, meaning dead pet if there's PvP AoE about).
Result: Jumping into combat as a BM hunter can mean instantly dead pet, unless you already had the pet out riding beside you. Switching a pet in combat or dismounting from a flyer into combat = dead pet, and thus often dead hunter. Using HotP does not help, since the pet is ressed at the low HP point and in the same spot it dropped. This bug can be counteracted to SOME extent through skill, but this bug would be far better off fixed than being one more game bug hunters need to work around (and in any case, savvy PvPers can and will switch to & instantly gank a newly-summoned pet).
Suggestion: FIX it. Please. This bug has been around for YEARS. It's simple, just make the base HP much, much higher and change the scaling to match.
I'd say this was a known issue that Blizz is working on, and ignore it, but honestly it's been around for so many years (and with no fix) that I now doubt that that's the case. Bug's still here, still crippling in certain cases, still needs a fix!