4 species have Plus as a hidden ability, 5 as a normal one

Effect, in PokéAPI's words

“Increases Special Attack to 1.5× when a friendly Pokémon has Plus or Minus.”

As a hidden ability

Species	Slot	Note
Mareep	Hidden	Since Generation V
Flaaffy	Hidden	Since Generation V
Ampharos	Hidden	Since Generation V
Dedenne	Hidden

As a normal ability

Species	Slot	Note
Plusle	Normal
Klink	Normal
Klang	Normal
Klinklang	Normal
Toxtricity	Normal

Elsewhere in PokéAPI

1 more holder without a page here

Alternate forms and other species that PokéAPI lists with Plus.

Toxtricity Amped Gmax