With the event "Rakghoul Resurgence", some successes have emerged, including some associated with seven Rakghoul pets. To achieve these seven successes, you will need to collect seven pets, then eliminate the Tunnel Ranger, a boss who patrols the tunnels under Alderaan with, each time, one of the little rakghouls by your side.
Here are the ways to get these seven different little rakghouls.
Little midnight rakghoul
You can find her in the Siege of Kaon Flashpoint, defeating the Midnight Rakghoul bonus boss, or in the Lost Isle, on Sentinel Droid LR-5.
Little pale rakghoul
You can get it from a "Frenzied Infested", an elite enemy that occasionally spawns near the OHNR base camp on Alderaan.
Little purple rakghoul
You'll get it from the Tunnel Ranger boss, who ... well, roams the tunnels under Alderaan.
Little fungal rakghoul
You will have a chance to get it on one of the rakghouls in the "Hey, I'm a Mold" achievement.
Little Symbiotic Rakghoul
You will be able to obtain it on the Amblyope, World boss instantiated from the event.
Small contaminated and irradiated rakghoul
You can buy these two rakghouls from Jeevlic, the Jawa merchant.
All you have to do is collect them and the achievements!