Tickers are small, fragile creatures that are used to deliver explosive charges by the Locust in the Gears of War series. Their namesake, their constant ticking noise, is the only warning soldiers have of their presence, and their rapid movement makes them neigh-unpredictable.
The bombs are detonated when next to an enemy, or can be discharged by gunfire, making timing crucial in most cases, as killing them while they're close can be just as deadly as letting them walk right up to the target.
Like most creatures the Locust use in combat, the tickers seen with bombs are tamed animals, and are not voluntarily having bombs placed on them, as evidenced by the struggling ticker seen in one of Gears of War 3's cut-scenes. Wild tickers have another layer of flesh atop them, and mostly live by eating metallic objects, such as ammunition and grenades. Killing them quickly can preserve consumed ammo, but will also detonate any grenades they have eaten.