Littlefoot is a brontosaurus, more properly called an apatosaurus. National Public Radio explains that the brontosaurus never actually existed. The first apatosaurus was originally reconstructed with the wrong skull. When complete apatosaurus skeletons with the correct skulls were later identified, they were mistakenly given a new name, the brontosaurus.
Although the mistake in naming was noted as early as 1903, it wasn't until 1979 that the brontosaurus was officially done away with when the Carnegie Museum in Pittsburgh replaced the head of its apatosaurus with the correct head. In the actual "Land Before Time" movie and TV episodes, Littlefoot and his relatives are referred to as "longnecks" by the other characters.