Pokemon Egg Chart

Here's a chart of which Pokemon hatch out of which eggs:

So for example, Chansey will only hatch out of a 10km egg. But you could get an Abra from a 5km egg or a 10km egg.
And a Zubat could come out of a 2km, 5km, or 10km egg.
So even if you have a 10km egg, there's still a chance you might just hatch a regular old Ratatta!

(Thanks to reddit user Taiyo4D for this awesome chart!)