Which one do I have to capture? As far as I can see, the Siren is the only appropriate alien.
But I researched her already.
Had the same problem. Had one siren captured, but needed to capture another one to be able to do that research. Bug IMO.
The only difference between those two was that first siren had pointed had (like horns) and second one had rounded head. But I think that doesn’t matter, just get another one.
Yes all Siren have at least one psy ability, the double capture is perhaps a design choice, but I agree it looks a bit weird. The research potential should appear later, like after one enemy with psy ability was captured.
EDIT:
Any Sentinel should also have one Psy ability. They don’t spawn in Haven defenses nor bases defenses nor ambushes. But they can spawn in Scavenging, nest/lair/citadel, some special missions. They are very easy to capture with long range ie the Neural SR.
You have to capture a Siren with the “Psychic scream” ability. If the first Siren you captured is the “Frenzy” one, bad luck.
Mmm looks like a bug, control is a clear psy ability and any Siren has it.