The ancient Egyptians used donkeys and oxen as beasts of burden. They plowed the fields and trampled seed into the soil. The slaughter of a fattened ox was part of an offering ritual. Horses were introduced by the Hyksos in the Second Intermediate Period, and the camel, although known from the New Kingdom, was not used as a beast of burden until the Late Period. There is also evidence to suggest that elephants were briefly used in the Late Period, but largely abandoned due to lack of grazing land. Dogs, cats and monkeys were common family pets, while more exotic pets imported from the heart of Africa, such as lions, were reserved for royalty. Herodotus observed that the Egyptians were the only people to keep their animals with them in their houses. During the Predynastic and Late periods, the worship of the gods in their animal form was extremely popular, such as the cat goddess Bastet and the ibis god Thoth, and these animals were bred in large numbers on farms for the purpose of ritual sacrifice.