For instance, the Warcraft Universe contains fictional races (various), such as the Orcs, Humans, Night Elves, Blood Elves, Tauren, etc. Are these races considered "original works" to fall under copyright law?
Yes, but …
Most fantasy races you named are part of long-standing folklore and any copyright the author had has long since expired. Orcs are from Germanic folklore, elves from Celtic, humans are, well, human and I don’t know what a Tauran is.
Now, the particular imagery and description applied to these races by Warcraft is covered by copyright. So generic orcs aren’t covered by copyright but orcs specific to Warcraft or Tolkien or Dungeons and Dragons are.