Every Pokemon is listed to a certain "Egg Group" that allows them to breed with any Pokemon that belongs to that same Egg Group. But some combinations are rather odd:

Wailord can breed with Skitty, despite Wailord being one of the largest Pokemon ever, and Skitty being one of the smallest.

Zangoose and Seviper are said to be mortal enemies and hate each other to the core. But, they can breed together.

But yet, somehow, someway, Nidoking and Nidoqueen are not part of the same Egg Groups and can not breed together.

...Nintendo and GameFreak are weird.