Screamers were a variation of SwarmJuvies. Seemingly malformed, if a Screamer was released from its pod, it would release a piercing screech that would rupture all other pods in the vicinity, likely freeing a pack of Juvies to overwhelm the perpetrator.
Screamers are limbless and incapable of attacking the player, and actually die after emitting their eponymous scream, as if they were too underdeveloped to exist outside their pod yet.
Screamers only appear in the campaign.
Strangely, though Screamers are presumably an underdeveloped or malformed Juvie, their facial structure is vastly different - their jaws run vertically down the middle of the face, as seen during the first Screamer cutscene in Gears of War 4. This might indicate that instead of being a malformed Juvie, the Screamer is a deliberately-grown organism, presumably for aiding in the defense of Hives.