One of the most important things about a game is the realism of names. We have naming guides for the player races—and those serve us well—but monsters are a different story. Monsters are the most interacted with component of a game world outside of other players. You'll visit Dulari's camp once or twice, but you'll be giving ghilliedhus a beatdown for years to come—which means plenty of time to think, "Why that name?"
So, what's in a name? Names that are thematically consistent and distinctive, that have some sort of meaning (preferably one that's emotionally evocative), make the world feel more real. For TERA, a world with an entirely new palette, that's quite a challenge. A tuwangi devastator might sound like a credible foe on paper, but one look at the cute, little guy and nobody will buy that as a name. However, they'll remember that time they faced down a dozen tuwangi blockheads.
For monsters that you see in a lot of different forms, there needs to be a system. Orcans and kulkari are proud, tribal groups and it's easiest to identify them by tribe. This way, you're not running into orcan warriors over and over. This one is from the Red Fang tribe--rather different than the Dark Claw tribe, yet familiar and understood all at once. And it's good to know at a glance that a monster wielding two axes is always going to be a reaper or reaver (and to know it's time to be scared and summon a thrall!).
At the same time, variety really is the spice of life. You learn to love your thesaurus, and when that fails, you start digging into other languages. Tuwangi, for example, was an Aboriginal name Bridget McKenna found. It fit and we loved it. Other times, another language can let you "label" something while still making the name fresh. Latin, in particular, is a useful language to mine because it informs so much of the modern English lexicon that the meanings often come through without someone googling it. (I heart Latin.)
Sometimes, however, all it takes a crazed writer's brain, some weird syllables, and a sense of when the word is "right" to name something. The argons are alien and otherworldly. Since I wasn't allowed to call them all Eggplant Evil This or Eggplant Evil That, I had to make up names. Again, they need to sound consistent-ish and related, but also creepy and a tad scary—and not Tolkien rip-offs. A fortress worker is much less worrisome than a chaogg slasher, to say nothing of an iknephar darkweaver.
Monsters aren't just loot piñatas dropped into the world for your destructive joy, they, and their names, are another avenue (however tiny!) to deliver what really matters to us (and, we hope, you!): A story.