Ponce da Quirm is no relation to Leonardo da Quirm (da Quirm simply meaning 'from Quirm'. He went in search of the Fountain of Youth and spent all his life in the search, prompting Rincewind to ponder whether he'd have been better off living his life instead...
He was encountered by Rincewind in Eric, deep in the Tezuman jungles, captured and due to be sacrificed, still searching for the legendary Fountain of Youth.
He reappeared later in the same book in Hell, still accompanied by the possessed parrot which Rincewind presented to him a hate-present.
Ponce da Quirm's name and life are based on the Roundworld Juan Ponce de León who was a was a Spanish explorer and conquistador born in Santervás de Campos, Valladolid, Spain in 1474. Though little is known about his family, he was of noble birth and served in the Spanish military from a young age. He first came to the Americas as a "gentlemen volunteer" with Christopher Columbus's second expedition in 1493. Pratchett has likely used him in Eric for the comedic value of his first name - Ponce meaning an effeminate man.