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.

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

Unlock a variety of weapons in Roblox's Zombie Game. Learn strategic tips, acquire powerful weapons, and dominate the undead onslaught.