OK once you get the thorn-laden heart (bear, stag, saber), you take them to Athainne in Drustvar, she gives you Moontouched Thorns to take to Ulfar in the cave, He then gives you the corresponding item Thornspeaker Ritual Knife, once you have one of the 3 go back to emerald dream and look for Agyr (Stag) at 44, 63, Astera (bear) at 36, 61, and Lilliuna (saber in cave) 57, 48 (reading the item tooltip will tell you which Knife you have, you kill the spawned mob, use the knife on the corpse and tame the next spawn to get your thorn beast. Hope this clears up the confusion (Handynotes: Dragonflight Misc option will show the spawn points for each) ( there are 3 different color combos for each one)