There are not any cards that are non-land and a Plains. This is because each type has their own list of subtypes that can be associated with them and Plains is a land subtype. Cards cannot have subtypes that don't match one of their types, therefore it is impossible to have a card that is a Plains but not a land.
205.3d An object can’t gain a subtype that doesn’t correspond to one of that object’s types.
205.3i Lands have their own unique set of subtypes; these subtypes are called land types. The land types are Desert, Forest, Gate, Island, Lair, Locus, Mine, Mountain, Plains, Power-Plant, Swamp, Tower, and Urza’s.
Of that list, Forest, Island, Mountain, Plains, and Swamp are the basic land types. See rule 305.6.
Also as had been mentioned the second sentence of the Shepard's ability should be "If that land is a Plains, you may return that nonland permanent card to the battlefield instead." referring to the land that triggered the ability in the first place, not the card you target in your graveyard.