What is the young one of horse called as?

Colt

The answer is (☀) Colt.

A colt is a male foal, or a young horse under the age of four. A female foal is called a filly. The term "foal" can be used for both male and female foals, but it is more commonly used for foals under the age of one year.

The other options are incorrect. A calf is a young cow, a kid is a young goat, and a cygnet is a young swan.