Flesh Beasts are the offspring of Flesh Spawners. The Dark Wanderer seemed to have an affinity for these creatures and summoned them from Hell on several occasions. Although nasty and dangerous on the mortal plane, they are little more than a nuisance in the Burning Hells.
Flesh Beasts are seen in Diablo II's opening cinematic and are later encountered during the Dark Wanderer's brief cameo at the start of [[[Act III]], just outside the Kurast Docks. Although they might present a bit of a challenge at that point, by the time they become commonplace enemies in Act IV, the only dangerous things about them are their numbers and their parents.