A colt is an intact (uncastrated) male horse under four years old. The term marks age and sex, not training. At four a colt becomes a stallion, or a gelding if castrated. In casual use a young male foal is called a colt, but precisely the word covers the male horse through its growing years.
See also: what an intact colt becomes at four, what a colt becomes when castrated
Further reading: Colt (horse) on Wikipedia; management of reproduction in horses from the Merck Veterinary Manual.