Yep, this exists. This Star Wars AT-AT Walker Dog Costume is basically a doggie hat and jumpsuit. There’s a head-wrap that has an elastic band. The body’s all one piece, so you only have to fasten some hook-and-loop underneath your pet, sorta like a saddle girth.

Star Wars AT-AT Imperial Walker Pet Costume
Officially-licensed Star Wars merchandise
Choose Small, Medium, Large, or X-Large
Head wrap + one-piece jumpsuit
Materials: Polyester with polyurethane foam
Care Instructions: Hand wash in cold water and dry flat