They are the souls of overweight neckbeards, reincarnated to slap you with their 8th chin and crush you with their man boobs . Do not succumb like so many others

Giant space turtles that come from a planet with tremendous gravity, and pressure. Which Is why they need to live in pressurized suits off of their home world. Makes for a nice POP when you get a critical :)

The Cabal are what happen when a bunch of Volus take too many performance enhancers

Each race is a living personification of humanity. The Fallen personify the Feudal Age leadership style, using Lords, Kings, and contesting Houses. While they aren't with each other, they aren't against each other either. They also have a strong honor code, as seen with the Dregs. The Hive are ritualistic Shamans and Witch Doctors. They personify the use of Voodoo, Necromancy, and Sorcery. The Cabal personify the Roman Age, and the Cesarian era. They maintain the belief of the "perfect soldier" and the belief that the leader must be the strongest of the soldiers (as seen with the Colossi and Ta'aurc). The Vex are a double-personification of the "mythological" outlook of the future, where robots are basically all-ruling. The age-old "robot rebellion" like Terminator.

Vex are cyborgs

I don't know if you'll get direct analogs with these creatures. The Vex are more than straight up robots, as evidenced by the blurb for the New exotic Hunter arms. Or how they have liquid neural systems. Or something. I wouldn't say Hive are very insect like either... Insects in my mind have multifaceted eyes, but not always more than two. I bet we'll get more details in the coming years. Either way, I want to know more'

Cabal are space turtles.