SB 4.19.11

carameṇāśvamedhena
 yajamāne yajuṣ-patim
vainye yajña-paśuṁ spardhann
 apovāha tirohitaḥ
Synonyms: 
carameṇa — by the last one; aśva-medhena — by the aśvamedha sacrifice; yajamāne — when he was performing the sacrifice; yajuḥ-patim — for satisfaction of the Lord of yajña, Viṣṇu; vainye — the son of King Vena; yajña-paśum — the animal meant to be sacrificed in the yajña; spardhan — being envious; apovāha — stole; tirohitaḥ — being invisible.
Translation: 
When Pṛthu Mahārāja was performing the last horse sacrifice [aśvamedha-yajña], King Indra, invisible to everyone, stole the horse intended for sacrifice. He did this because of his great envy of King Pṛthu.
Purport: 

King Indra is known as śata-kratu, which indicates that he has performed one hundred horse sacrifices (aśvamedha-yajña). We should know, however, that the animals sacrificed in the yajña 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 yajña. When King Pṛthu was performing one hundred yajñas, Indra became very envious because he did not want anyone to excel him. Being an ordinary living entity, he became envious of King Pṛthu, and, making himself invisible, he stole the horse and thus impeded the yajña performance.