You can get this along with Nom Nom Nom!
First off play and complete the Unwarranted Aggression mission which acts as a tutorial on how to use Wretched Graugs. Once it is completed you will still be mounted on the one from the mission and can use it in free roam. Now look for captains by using Wraith Vision and engage one. Try to stay away from captains holding bows and arrows as they can easily dismount you. To get Wretched Retch hold the left trigger to aim and right trigger to vomit on the captain. Nom Nom Nom! is a bit trickier. You can either find a captain with a fear of Graugs who will flee on site and chase him down to eat him or you can dismount kill the orcs and weaken him. Then remount and eat him up.