What is the opposite gender of colts?

Answer 1

There are two genders - male and female. Although certain genetic disorders and mutations can alter the characteristics of these genders, there are no genders other than male or female, which are opposites. "Colt" is neither "male" nor "female"; therefore, gender wise, "colt" has no opposite.

Answer 2

The complimentary sex to a colt is a filly. Horses do not understand the concept of gender though and therefore there is no "opposite gender" to a horse or any animal including humans. Gender is a part of a humans identity and there is no opposite to that. Sex on the other hand refers to chromosomal characteristics. While sex for many animals includes XX and XY but there is much more to that including XXX and XXY.