The cow character who often appears alongside Mickey Mouse and Minnie Mouse is Clarabelle Cow.
Clarabelle Cow is one of the earliest recurring characters associated with the Mickey Mouse universe. She first appeared in the 1928 short Plane Crazy, the earliest produced Mickey Mouse cartoon, alongside Mickey and Minnie. Her early appearances established her as a tall, expressive cow who could serve as a friend, comic participant, or background character.
Clarabelle is sometimes confused with other Disney animal characters, especially because her role and relationships have varied across cartoons, comic strips, and later television productions. Her appearances have included stories with Horace Horsecollar, another early Disney character.
Unlike Mickey and Minnie, Clarabelle has not always appeared consistently in every era of Disney animation. Nevertheless, her long history reaches back to the earliest Mickey cartoons, making her an important part of the original supporting cast.