I cannot write anything better than this excellent and comprehensive wowhead guide, so please visit there to get all the detailed information.
The pets are easy to get — much easier than I previously anticipated. I thought they’d be purchases with Tricky Treats but they are in fact wild pets that show up in your Garrison when you decorate your Garrison with “Creepy Crawlers” from the vendor that sits near your fountain. It costs only 5 Spooky Supplies, which you can get every day by doing the daily quests that they give. It’s quite easy!
I recommend a S/S Spectral Spider breed for a solid PvP pet! The other two, Ghost Maggot and Ghastly Rat breed is negligible. I’ve been having great luck playing my current Halloween themed PvP team; Graves, Creepy Crate and the new Spectral Spider.