Grandma and Grandpa Longneck are Littlefoot's grandparents in The Land Before Time franchise.
They are each Apatosaurus, which is one of many species referred to as "Longnecks" by the characters in the franchise. They are Littlefoot's maternal grandparents - due to being the parents of his mother - and his primary caregivers after their daughter's death. They act as parents to Littlefoot; giving him advice when needed, and take the responsibility of protecting him very seriously.
Their real names have not yet been revealed, as nearly everyone in the Great Valley refers to them as "Grandma and Grandpa Longneck"; even each other, and Topps (a.k.a. Mr. Threehorn) continually refers to Grandpa as "Longneck". Bron calls Grandpa "Papa Longneck".