A ragged colt may make a good horse.

French: Méchant poulain peut devenir bon cheval (literally: A ragged colt may make a good horse)
Spanish: De potro sarnoso buen caballo hermoso (literally: A scabby colt may make a good horse)

Meaning: Unpromising beginnings don't determine a creature's potential; proper care and time can transform rough youth into valuable maturity.

Origin: From 'A Dictionary of English Proverbs and Proverbial Phrases' (1870). Public domain.