My three Rangers all have their Heart of Thorns exclusive pets now. It was not as hard as I thought it would be. The descriptions talk about some of the pets only being available after some events in the game. Turns out that it’s not that hard once you are playing in that area to participate in the events and then you just basically walk up and get the pets.
There are four HoT exclusive pets:
You can easily look these up on the Wiki pages and see their descriptions and a short description of how to get them.
Don’t be intimidated by the instructions. The Bristleback is the easiest to get. In Auric Basin, run South from the Bristleback Chasm WP to the glade where you find a couple of Veteran Bristlebacks and some Juveniles. Run in and charm a pet and run out. Even if you die on the way out, you have your pet.