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.

The Pain rework has to be one of the best fruit reworks ever done in Blox Fruits, thanks to the hidden abilities that players ...

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