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Matthew Medina has a series called Design Deconstructed where he talked about content he created for GW2 to serve as a portfolio after he got layed off.
Back in the day I commented about what the purpose was of this random confused cow was in the Labyrinthine Cliffs and this was his reply:
@MatthewMedina 5 years ago AH, the good old confused cow! So it actually served two purposes. It began when the art director was giving the team feedback, and asked if I could place a piece of content up there to reward/encourage players to explore up that way, since they were able to reach that plateau on their mounts. And I thought a bit about what I could put up there, and I talked with Connor Fallon, one of the living world designers and together we brainstormed up the idea of seeding a tease of the upcoming Living World episodes (wherein Kralkatorrik was in the mists, and opening portals into Tyria). We worked up this story of a cow that had wandered into one of these portals, and come out the other side into Labyrinthine Cliffs. So I put the cow there as a tease of Season 4's story - the cow made a return in Jahai Bluffs as well. I had planned a whole resolution to this little story, but sadly didn't get the chance to add it to the game.
So there you have it the original purpose of the cow, maybe his story will get resolved at some point now that Matthew is back at the company (eventhough he's currently working on an unannounced project instead of GW2)