4 species have Aftermath as a hidden ability, 4 as a normal one

Effect, in PokéAPI's words

“Damages the attacker for 1/4 its max HP when knocked out by a contact move.”

As a hidden ability

Species	Slot	Note
Voltorb	Hidden	Since Generation V
Electrode	Hidden	Since Generation V
Trubbish	Hidden
Garbodor	Hidden

As a normal ability

Species	Slot	Note
Drifloon	Normal
Drifblim	Normal
Stunky	Normal
Skuntank	Normal

Elsewhere in PokéAPI

3 more holders without a page here

Alternate forms and other species that PokéAPI lists with Aftermath.

Garbodor Gmax (hidden), Voltorb Hisui (hidden), Electrode Hisui (hidden)