Mid 19th century. From Spanish potro colt, foal (940; 924 as poltro), apparently ultimately from a post-classical Latin derivative of classical Latin pullus foal, although this presents phonological difficulties.
Definition of potro in US English:
potro
nounˈpɒtrəʊ
In South America: a colt, a young horse; a pony.
Origin
Mid 19th century. From Spanish potro colt, foal (940; 924 as poltro), apparently ultimately from a post-classical Latin derivative of classical Latin pullus foal, although this presents phonological difficulties.