A group of Longnecks in Long-neck.

"Mother, what's a Long-neck?"
"Why, it's what we are dear."

Littlefoot and his mother.

Longneck is the Land Before Time term for sauropods. It obviously comes from the fact that most of them have very long necks. Most species had very long tails as well, and all were huge. Species seen in The Land Before Time are:

Owing to the fairly level head possessed by some types, the racial slur "Flathead" is sometimes used by other dinosaurs when referring to a Longneck (possibly only Apatosaurus), or also "Bumphead" when referring to sauropods such as Diplodocus. However, Petrie uses "Flathead" seemingly as an affectionate nickname for his friend rather than an insult. However, Littlefoot doesn't like being called Flathead.  

Legends about Longnecks

The Land Before Time VI: The Secret of Saurus Rock

The Lone Dinosaur was a legendary hero long ago - before Grandpa Longneck was alive, as he states later in the movie - who would just show up out of nowhere whenever there was danger afoot in a community. One day, what was declared the "biggest, meanest, most ferocious Sharptooth ever" came to the Great Valley, and while everyone ran away, the Lone Dinosaur appeared and battled with the Sharptooth on his own. His opponent managed to lacerate his eye, which apparently left a permanent scar running down the side of his face, but he ended up victorious.

A short while after he had defeated the Sharptooth and left the Great Valley, a rocky formation rose up out of the ground, shaped like a Longneck, with a ring of teeth encircling its neck. The teeth were said to be those of the same Sharptooth as the one he had killed. It was also believed that if the formation got damaged in any way, it would unyield its curse on the entire valley.

The Land Before Time X: The Great Longneck Migration

Pat tells the children a story he had heard from his grand-dad about how Longnecks came to have long necks. He claims that they are so if the Bright Circle ever falls, the Longnecks will be able to catch it.

"The Legend of the Story Speakers"

In "The Legend of the Story Speakers", Grandpa Longneck and Saro tell the children the story of how longnecks got their necks. The story states that they originally had short necks, to eat the leaves of the then-short trees. The trees "sang" to the Bright Circle (sun), and it pulled them up to hear their songs better; thus making them grow. This prevented the Longnecks from eating the leaves, and the Night Circle (moon) comforted them by shining a soothing light. They stretched their necks to see the light more closely, and thus their necks grew longer. The Longnecks were then able to eat the leaves once again.

Longneck individuals