Pepi is a poor Spanish boy who temporarily became Homer's son.
History
When Bart joins the Bigger Brothers non-profit organization and is assigned to Tom as his temporary father, Homer signs up to the organization to get a temporary son as well in order to get revenge on Bart, and eventually gets Pepi. As Homer is his temporary father, he learns nonsensical things, such as the fact that whales are not mammals. Later, after Homer and Tom get into a fight, Homer is sent to the hospital (due to his spine getting cracked when he bends onto a fire hydrant), leaving Pepi without a father and Tom without a son. Bart realizes that Pepi is more deserving of Tom than he is, so he pairs them up, and they walk into the sunset as Pepi officially becomes Tom's son.
Appearances
Episode – "Brother from the Same Planet" 
Episode – "My Fare Lady" (opening sequence)