Playing a 1.6 Game holding a horse egg, saving, and then playing into 1.5.2 results into holding a plain spawn egg.

That is intended... as their is no Horse egg in 1.5.2 so its just going to be a generic spawn egg.

Works as intended just as Th3F4114n0n3 said.
This happens because spawn eggs use different damage value and unexisting ID will automatically give you a plain spawn egg.

No this isn't invalid this is works as intended.