Imago were SwarmDrones that recently matured from Juvies from inside a nest. Emerging pale and naked, they scavenged whatever weapons were at hand and charged at their enemy, lacking the sense to remain behind cover for long.
Imago possess all of the capabilities of your standard Drone, but have slightly less health and fight more aggressively, eschewing cover for rapid aggression. You don't need to prioritize them over other Drones, but you shouldn't ignore them either.
If you seal a nest with an explosive while Imago are still emerging from it, you'll kill them instantly, regardless of health. Frag grenades are your best option for this, due to their availability; always keep a few on hand for this purpose.
Imago is the final development stage of an insect after undergoing metamorphosis.
Imago are somewhat uncommon foes outside of their set appearances in the campaign, but they may be fought in Horde mode in Gears 5. They appear during the "drone" themed wave sets in the earlier waves, usually alongside Juvies and Rejects.
The Imago is an unlockable character for the Swarm in Gears of War 4 multiplayer. A Spectre skin is only available by crafting scrap, though there was a chance of receiving him early in the Carmine, Palace Guard and Wrestler Oscar Gear Packs.