Getting into nerd stuff now

"Xeno" as a prefix means "guest" or "stranger". If I am remembering correctly, Gearbox has stated that the Race X isn't classified together based on genetic similarities like other animals, but on the common ability they have, being innate and instant teleportation.

"Phylum" is a taxonomic term used for classifying species, but more specifically, key differences they have that differentiate them from different members of their species. Considering how vastly different the Race X are with their abilities being unheard of among other creatures, this would make sense. At least to me.

So Race X is short for Xenophylum.

Why did I go this in depth about science stuff? Well "Race X" sounded too generic for these entities so I wanted to spice the name up a bit to make it less bland.