Rawrvek is the pet version of Gorvek, the dragon riderVindicta's mount. Rawrvek is unlocked by inspecting the Glimmering scale, which can be obtained as a drop from the boss Vindicta and Gorvek. The odds of receiving the item are 1 in 2000 (normal mode), or 1 in 1000 (challenge mode), both with a threshold of 400 (explanation of drop mechanics). This is not affected by the increased drop chance reward from reputation. Being a dragon pet, Rawrvek requires level 99 Summoning.
The internal 'item' name for the player-following version of Rawrvek (as opposed to the 'item' that appears in the Menagerie) is Gorvektimus prime, a reference to Optimus Prime. This name is unable to be seen in-game and is presumably an early name for the pet.