If time is fluid and we assume that Pony Land of G1 is actually modern day Equestria. Then what happened to the Moochick, and more importantly what happened to Flutter Valley and the Sun Stone.
I can imagine that the Crystal Desert became the home of the Crystal Ponies.
But what happened to the Sun Stone?

Could you tell me about the Sun Stone? Sounds interesting.

4500929
That is an interesting conundrum, I would probably assume that the Flutter Ponies hid Flutter Valley, thus the Sunstone, from Equestria. The question turn from what happened to why it happened. And honestly the the first thought that comes to mind is industrialization

4500960
The Sun Stone is the magical artifact that keeps Flutter Valley warm. If it ever goes missing for a full day this flower filled place turns into an infertile swamp filled with dead plants. It makes Flutter Valley the idea place to live for this rare species. Flutter ponies depend on it so much they have a holiday to celebrate it.
(At least as far a cannon goes)

Ignoring the comic, sounds like a great genesis story for changelings.