Diplodocus

The Diplodocus is a five-part fossil that can be donated to the museum in.

Completing the Diplodocus in New Leaf
"The diplodocus was a herbivore of the late Jurassic period sporting a tremendously long tail and neck. The neck was so long that, from snout to tail, its length could reach a staggering 175 feet long. Evidence suggests it would use its long, slender tail like a whip to defend itself from attackers."-Blathers

In real life
The diploducus had a long neck and live 162 million years ago. It was a herbivore and couldn't actually chew.