A young horse is a foal. Foals are born to a mare, a sexually mature female.

VOA: special.2009.07.21

"This exhibition represents a more mature view,if you like, one which can take account of catastrophes like earthquakes, problems at nuclear plants and tsunamis."

VOA: special.2011.04.08

A sexually mature male is called a boar.

VOA: special.2009.07.21

South Korean Foreign Minister Yoo Myung-hwan told lawmakers Tuesday joining PSI was the correct move for a "mature nation."

VOA: standard.2009.05.26