Walkers

Multiple walkers appear as cameos in Rick Grimes vs Walter White. They are seen being shot by Rick in his second verse. They are played by Neil Blan, Amy Bury, Ray Timmons, and Tom Walsh.

Information on the Cameo
Zombies, usually referred to in The Walking Dead universe as "Walkers", are the eponymous main antagonists of The Walking Dead.

Zombies are undead creatures. They are bodies of deceased humans that have in some way been reanimated and move about, though lacking a brain or vital functions. Therefore, they are not considered to be alive. In most cases, they are depicted as mindless corpses with a desire to eat human flesh in order to survive.

In The Walking Dead, everyone carries a pathogen that can bring the dead back to life. After a person dies under any circumstances, the pathogen will reactivate parts of the brain that support necessary vital systems, and it will reanimate the corpse, thus making it a walker. Because only critical parts of the brain are activated, Walkers are known to have no memory of their previous selves (only a physical resemblance remains) and do nothing but attack and eat the living. Although Walkers don't form a real threat as individuals, they are especially dangerous when traveling in a large group or "herd". The only way to truly kill a Walker is by damaging its brain functions. Because they are literally rotting from lack of bodily activity, Walkers typically have decaying flesh and raggedy clothes covered with blood.

Appearance in the Rap Battle
A short clip of a walker was shown in the April 15th Monday Show. They appeared in the battle and three were killed by Rick Grimes, another came torwards Grimes, but he dodged it to let it attack Walter, who distracted it by throwing meth. It is possible they were attracted to Rick and Walter from the sound of their rapping.

Trivia

 * They are the fourth cameos to get killed, after Urban Ninjas, Mustached Cowboys, and Blackbeard's Pirates.
 * Coincidentally, all of them are groups of cameos.
 * Not all of the group is killed, however. Ray Timmons' walker appears to survive.
 * They attended the Heroin Joke's funeral in the Behind the Scenes, where they sang Amazing Grace.
 * In the Behind the Scenes, it was Neil Blan's walker who chased after Walter White, but in the finished battle, it was Ray Timmons.
 * Walker 2 (Ray Timmons) is the seventh person to cross the "border" in a battle. The others are; John Lennon, Adolf Hitler, Mustached Cowboys, Martin Luther King, Jr., Gandhi, Darth Vader, and Michael Jordan.
 * He is the second cameo to do this.