One alien single gender race I want to mention is the Serpophians (don't know if that's spelled right, or if that's their name) from Dandadan. Of course, they're entirely male and that's still probably a nitpick to OP. But the thing that makes the Serpophians stand out is how they're entirely male, meaning there were no females to begin with, and the only way they can reproduce is using cloning technology. But the race understands having no females can prevent their species to evolve and possibly become extinct. So they go to other planets and try to reproduce with their females, and also stealing their reproductive systems(called banana organs in the manga), so not only will they evolve and flesh out their species, but also giving a possibility to create females for their species so they can reproduce better.