An Elite Goon that has several of its minions fight alongside it. For example, an Evil Overlord may call upon his guards, a mother alien monstes may order its offspring to attack, a magic-based enemy will summon otherworldly creatures, a wizard will create clones of itself, and so forth.
Taking advantage of these extra baddies may be required to beat the boss. For example, the dtory may require the hero to take them out before you can even hit the boss at all, or he may have to trick them into attacking their leader.
Only rarely do the minions actually stay dead. Defeating all of the minions will cause the boss to revive them, or summon more to take their place. The hero can usually exploit this to their advantage by defeating all but one minion; other times the boss will simply revive any fallen minion, and defeating the minions merely just distracts the boss from attacking the party directly. And then there are times where the minions simply respawn on their own, allowing the boss to focus all of its attention on you.
Alternately, the Goons may become much more powerful as its minions are defeated, unleashing stronger uber-attacks against the group. Be warned that the Goons may remain in this empowered state even while reviving its comrades, making the fight even harder.
While these extra enemies to deal with may ostensibly make the fight harder, they may also represent an unlimited source of health or ammunition pickups, or experience points as they are destroyed.
Sometimes, Goons are very weakly tied to normal enemies and it feels like a boss just appeared in the middle of the battle
In general, the best strategy seems to be to concentrate all the offensive efforts on the main goon, as destroying him will defeat the minions too, and there's no sense on wasting time and ammo on the minions when they're probably just going to come back anyway. This is especially true if the boss itself has no form of attack, and its flunkies are its only form of offence.