According to the script notes "Dirgo is a tough-looking humanoid who is weathered and worn enough to suggest middle-age. Though his appearance is feral, he will emerge as an intelligent, if inflexible, being. Ultimately, he is self-serving; Dirgo looks out for Dirgo, because he's always had to.
StarTrek.com called him a "Pentaran shuttle pilot".  It's unclear if "Pentaran" is the name of his species or just refers to him being associated with the Pentarus system mining operation, especially since there was no hint of a native population in the system.