Well you may look at its spots and if they are starting disappear it is probably an older fawn. You may also be able to tell by the size of their hooves or antlers, if any antlers. With dogs they...
If it's a fawn, then it's a deer, and adult male deer are bucks.If this question is about horses, then the young is a foal, and an adult male is either a stallion, if intact, or a gelding, if...