Find the odd pair from the following options:

Let's analyze the relationship between the two words in each option:
* Dog : Puppy $\to$ Puppy is the offspring (young one) of a dog.
* Cow : Calf $\to$ Calf is the offspring (young one) of a cow.
* Sheep : Lamb $\to$ Lamb is the offspring (young one) of a sheep.
* Tortoise : Turtle $\to$ Tortoise and Turtle are two different distinct animals/reptiles belonging to the same order Testudines; one is not the baby version of the other.

Therefore, 'Tortoise : Turtle' is the odd pair.