There aren't enough animals on Okoto.
The Aktoros, or “dune bull,” is a massive animal that lives in the Region of Stone. They live among the dunes and use them to help protect themselves from the sandstorms; hence, their name. These enormous animals can be tamed, but it requires much time and patience. Their tempers can be short, and their hind legs and hooves are extremely strong. This is not necessarily a good combination, as they can split boulders apart with a single kick. It would not be entirely pleasant to have one attack you.
In addition to their short tempers and immense strength, the Aktoros are known for their notoriously long lifespans. In fact, they live for so long that if one dies under the care of its owner, it is considered to be a bad omen. Interestingly, many of the bulls died off shortly before the skull spiders arrived on the island. There were few bulls left on the island when the Toa arrived, and their numbers have not increased much. Perhaps when the evil has been vanquished, they will return in their numbers of old.