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 spawn point and AFK area in Zombies RNG (Image via Roblox) The first thing every player needs to do in Roblox Zombies RNG is to start rolling for weapons and get their hands on a decent rifle.