A vey few exceptions, namely the guild herald and guild page, as well as the Blizzard store moonkin. The moonkin only works with the corresponding faction, so if by chance you have a horde toon on one server, and an alliance toon on another server, they will not be able to battle with, or even summon the opposing faction version of the pet. However, if you have either the blizzard store gryphon or wyvern, they both are able to be summoned and able to be used in battles cross faction.
The guild page and herald are not able to be summoned by any toons in a guild that does not have the achievement, but neither can pet battle, only be summoned as a short term vendor on cool down.
Last edited by Mykro9
on March 15th, 2013, 6:13 pm, edited 1 time in total.