Recycled Playsets and Accessories
Hasbro reused molds for playsets and accessories within generations, across generation and across toy lines.
Hasbro reused this playset (with some modifications) for the Littlest Pet Shop line in 1995.  Notably, it does not include a music box / wind up feature.
Petite/Moon Dreamers Star Picks
Hasbro used the same mold of star picks for G1 Petite Ponies and Moon Dreamers. They differed in that one distributed with the Moon Dreamers were painted with a shiny coating and those for petite ponies were not.
The pony schoolhouse from G1 was reused in G2.
The G1 Show Stable was re-released during G1 several times.
German Show Stable, "Pony Haus"
Family Convertible/Pink and Fabulous Pony Car
An modified version of the car mold was used in both G3 and G4.