The mare's legs made now are manufactured as pistols, so they can have short barrels.
A shotgun would need an 18" barrel to be legal. Anything shorter and you're into AOW (Any Other Weapon) territory, requiring additional steps for ownership.
The closest readily available mare's leg type shotgun would be one of the many pistol grip pumps on the market.
You could cut an 1887 Winchester lever action shotgun clone down to mare's leg dimensions, but would need to jump through some legal hoops.