Which of the following is odd one out?

Horse: Cygnet

The odd one out is Horse: Cygnet.

In the other options, the first term is the adult animal, while the second term is the young of the same species.

• Sheep is the adult animal, while lamb is the young animal.
• Goat is the adult animal, while kid is the young animal.
• Buffalo is the adult animal, while calf is the young animal.

However, a cygnet is the young of a swan, not a horse. Therefore, Horse: Cygnet is the odd one out.