The best theory I have to their origins in the pokemon world
It would make sense considering their counterparts are aliens
Reaper evolution stages:
1st Oculus (maybe separate species with 1st stage as oculus and 2nd oculus from space battles)
2nd Destroyer class (This is Rezaras I assume) evolves into either:
3rd branch 1 Destroyer class with Hades cannon on back (end of evolution branch)
3rd branch 2 Sovering class
4th Harbinger (mega evolution)
1st stage based on her hologram from ME2
There could actually be 1st stage in form of Combat drone (Tali's Chatika), that branches out (like Eevee) into Prothean VI, Avina and EDI's 1st stage I mentioned above.
There are also several non holographic drones from ME1 that could be used for something I guess
3rd stage branch 2 Collector leader (Maybe Mega Evolution)
Praetorisect could maybe have some armour since it's made out of metal in the games?
Harvester: Reaper version
Reaper version of Rachnid. Ravageist either evolves from this or separately from Rachknight.
Rachni could maybe even have egg stage but that might be going too far.
should be previous stage for Zombrute evolution.
should be 2nd evolution branch from Phannibal.
And of course an absolute must have as suggested by gergory97 one and only, holy SPAAACE HAMSTERRRRrrrr XD!!!!!
Maybe as 1st stage for Pixik? It does look rodent like so it wouldn't be completely illogical :/ :)