Horde was the most popular multiplayer mode from Gears of War 2 so it seems natural that they'd add to it in Gears 3. However, instead of merely tweaking Horde mode, Epic Games is adding a whole new way to enjoy the action.
Horde had five players fending off waves of assaulting Locusts. In the new Beast mode, you play as those Locusts. You and your comrades must go on the offensive and wipe out human soldiers controlled by the A.I.. You must kill the entire wave of humans before the timer runs out or your team no longer has any tokens.
The squad starts out with a large pool of tokens that you spend in order to "buy" the unit you control for your next life. The units range from the kamikaze Ticker to the beefier Boomers and Grinders. The amount of tokens needed to purchase a unit depends on how powerful they are, so naturally the thin-skinned Ticker's the cheapest option. You earn tokens by killing enemies and surviving rounds.
The diverse capabilities of these units makes Beast mode into a sort of class-based action game. When playing as humans in other multiplayer, the emphasis is on equipment but here the focus is on the Locusts' unique abilities. Each of the Locusts has about two or three abilities that make them suited toward one type of combat situation. For example, the Butcher is great at carving up humans in close quarters with his knife but he's a big target and moves pretty slowly. When trying to attack humans in an area with little cover, you may instead opt for a more nimble class like the Kantus, which are armed with burst pistols and ink grenades. There's no "one size fits all" unit, really.
Your choice of unit really changes the nature of the game altogether. The Ticker can only withstand minimal damage so he needs to sneak up on his prey. You'll have to dart from cover to cover and wait until your target is distracted by your friends before you can run up and self-destruct in his face. If you're playing as a Berserker, though, you can just charge enemies head-on and pummel them. However, you're nearly blind so you have to rely on cues (like gunfire from enemies/friends) to snuff out the enemies.
The Locust units' narrow abilities teamwork becomes more necessary. The COG soldiers will bust out heavy armaments like the Hammer of Dawn and turrets so you'll want to attack en masse. Lone wolf tactics don't really work here. You won't get a lot of players off in the corner sniping the whole time like you might with Horde mode.
The point of all this is this: Beast mode isn't just Horde mode with Locust skins. It's a completely new experience that turns the game's co-op on its head. Whether Beast will be played as profusely as Horde (which will probably get its own facelift in GoW3) remains to be seen. However, the deep bench of Locust units and the different strategies they demand of the player should entertain even the most jaded Gears fans for a long while.