Rick's foal is an chud-human hybrid offspring of Rick Sanchez and Princess Poñeta.
The result of a forbidden love between his parents, which caused the King - his grandfather - to sentence Rick to death. The war was ended when Poñeta revealed she actually loves Rick and got pregnant by him.
With the union of humans and CHUDs, they slayed all of Morty's mutant sperms before conquering earth. Right after defeating the monster sperms, Rick and Poñeta start to make out, but feeling a burst in her belly, Poñeta gives birth to their son. Right after being born, Rick's foal digs into the CHUD kingdom, Poñeta explaining that CHUD foals are self reliant from birth, but after giving birth to Rick's foal, she and Rick part ways.