7 species have Heavy Metal as a hidden ability, 1 as a normal one

Effect, in PokéAPI's words

“Doubles the Pokémon's weight.”

As a hidden ability

Species	Slot	Note
Aron	Hidden	Since Generation V
Lairon	Hidden	Since Generation V
Aggron	Hidden	Since Generation V
Bronzor	Hidden	Since Generation V
Bronzong	Hidden	Since Generation V
Cufant	Hidden
Copperajah	Hidden

As a normal ability

Species	Slot	Note
Duraludon	Normal

Elsewhere in PokéAPI

2 more holders without a page here

Alternate forms and other species that PokéAPI lists with Heavy Metal.

Copperajah Gmax (hidden), Duraludon Gmax