@JMP In context of Rahkshi, no. Is it necessary for a monster to have a gender?
Most creatures and monsters in fiction would, realistically, have genders. However, things are different when you look at it in the context of the narrative.
Monsters aren't supposed to be sympathetic characters. They're inhuman things for the hero to overcome. Yes, you can give them genders but it isn't required. What matters is the threat it poses to the hero(es) of the story. Do we really need to know what gender the Hydra was when it fought Hercules?
Most swarm villains are in the same boat. They were never meant to be people, just powerful creatures for the Toa to overcome.
(Also, one way to make creatures less repeatable is to strip them of gender identity for the audience. We identify with "hims" and "hers" but not "its." )