Instructions
Add 10 square pinometers each of snow and water to your garden. A pinometer is the same size as one section of fence, with the largest possible garden being 32 pinometers on each side for a total of 1,024 usable pinometers. Snow and water are two basic elements of your garden and are available from the beginning.
Wait for a Walrusk to appear. It won't show up until you're at least Level 7.
Add 10 more square pinometers each of snow and water, because the Walrusk will need 20 total square pinometers of each element to become a permanent resident of your garden.
Add two square pinometers of sand to your garden. This will cause a Custacean to appear.
Go to Costolot's General Store, and purchase three Bullrushes.
Place the three Bullrushes and one Custacean near the Walrusk. It will eat these four items, which will fulfill the residency requirements of the Walrusk.