Magic takes place in a big, big Multiverse, which draws upon a lot of influences but isn't beholden to them. Primarily, Magic is based on established fantasy tropes and the mythologies of real-world places, so it has plenty of creatures to choose from - Angels to Zombies, Satyrs to Werewolves. However, Magic's creative team also doesn't want to be derivative, and hence Magic has, among other novel elements, types of creatures that appear nowhere else in fantasy, like Atogs, Surrakar, and Lhurgoyfs. And Mark Rosewater has made clear on his Tumblr blog that this tradition is likely to continue.
When zeritanos asked, "Are you entirely closed off to the idea of making new races? Or are you only going to use established fantasy races for tribes?" Mark responded:
We still make up new races from time to time.
Since "unsolicited creature type ideas" isn't the same as "unsolicited card designs," commenters were quick to chime in - wreckedsickan said, "More salamanders and lizard men!! Spider people!!! CRAB PEOPLE!!!! Tastes like crab, but talks like people!" And the unfamiliar landscapes of Tarkir might be home to original creature types as well, starting this fall.