King Indra is known as sata-kratu, which indicates that he has performed one hundred horse sacrifices (asvamedha-yajna). We should know, however, that the animals sacrificed in the yajna were not killed. If the Vedic mantras were properly pronounced during the sacrifice, the animal sacrificed would come out again with a new life. That is the test for a successful yajna. When King Prthu was performing one hundred yajnas, Indra became very envious because he did not want anyone to excel him. Being an ordinary living entity, he became envious of King Prthu, and, making himself invisible, he stole the horse and thus impeded the yajna performance.