Quote haflingers4ever:EDIT: If I breed a Mer with an Earthpony can I get a Mer?
Since the earth pony is one of the breeds that are used to create a merpony, yes, you do have a chance of a mer. Half the time the mer will breed as a sea pony, and when it does, there is still the 20% chance of hybridizing with the earthie and recreating a mer. So roughly one out of every 10 babies would be a merpony. Breeding a mer with a sea pony will do the same.
And since mers are used to create the hybrid merdragons, you can also get a mer about 40% of the time when breeding a mer with a dragon.
If you breed a mer with a merdragon the mer will split to it's component breeds, but the merdragon will as well, so about 25% of the foals will be mers.
A reindeer pony, or any other I didn't mention above, will never have a mer baby when bred with a mer.
Breeding with mer ponies is about the most complicated, partner breed dependent breeding you can do since they are currently the only breed that is a hybrid and also a component of another hybrid.