You can buy this mount in the Affinity Rewards section of the store using Affinity Currency.
It is also occasionally available in the store for Credits, such as during this sale on May 14th 2020.

• While riding Opie your character will become very small and you will be given a temporary ability bar with all of Opies abilities..
  

• Artifact trackers – Tracking abilities for the 6 different types of artifacts, costing varying amount of Planar Pellets and all lasting for 5 minutes. You can only use one at a time.
• Tip Toe – Makes you move very slowly for a few seconds, costs 2 Planar Pellets.
• Squirrel Jump – Jump to a targeted location up to 30 meters away. Costs 35 Planar Pellets. The Squirrel Jump ability can be used to help you complete the Vostigar Peaks jumping puzzle.

• Each ability costs a set amount of Planar Pellets. You can buy Planar Pellets with Lucky Coins from the Artifact Collector NPC, you may also find some on the Auction House from other players, your minions will bring them back occasionally or you can loot small amounts them in open world while foraging/mining. You can also buy planar pellets in TOKS dungeon for TOK tokens.. it is 4000 TOK tokens for 1000 planar pellets.

• Opie was originally given out as part of the Starfall Prophecy expansion packages (Later renamed to Prophecy of Ahnket).