Last names? Only Pies and Cakes

[cider float 2,539]

So far the only ponies with last names that are given down by generations are the Cakes and the Pies. At first one would think this is an earth pony tradition but the Apple Family does not retain their last names because they don't have any.

[Frostgage 7,982]

Last names in mlp are definitely inconsistent like you pointed out. I think Twilight was referred to as "Miss Sparkle" once. Pony names are pretty liberal in general, they don't seem to follow any kind of rules

[Jammo 986]

It seems to be more a signifier of identity rather than identity. The only ponies who seem to use it like a surname are ponies that seem to have a strong familiar bond. Like cakes and pies. The rest have them as extensions of their first names.

[chirox the pony 1,336]

I think it's just a case of the toy designers coming up with names with no real regard for story. As a result the writers can only come up with matching surnames for characters that aren't already dolls. In a story sense I guess your surname is whatever you want it to be in Equestria. 

[cmarston1 5,965]

Well seeing how Equestria doesn't act like how the real world does than I think it is safe to assume that surnames don't either.