Roblox Grow A Garden has many things you can do, including apparently getting a chicken...who is also a zombie. Go figure.

Roblox Chicken is an interesting experience that tests your movement skills at every turn. Your primary objective is to steal the eggs and survive without being caught by the chickens.

If you log into Roblox: Grow A Garden after the event is already live, you won't receive the Chicken Zombie. As such, the only way to currently get this pet is by trading.