Megacerops

The Megacerops is a three-part fossil that can be donated to the museum in.

In New Leaf
"The megacerops was a mammalian grazer with a Y-shaped horn on its nose. It is a distant relative of the rhinoceros, which the horn helps give away. Its main source of food was soft grass, but climate change caused this food source to disappear. Ultimately, without a thriving source of fresh food, megacerops joined other creatures in extinction."

In New Horizons
When donating to the museum, Blathers will say the following:

"Hoo! I had been hoping someone would bring me a Megacerops! This large fellow was a bit like our modern rhinoceros, but with two horns on its nose! Sadly, their small teeth restricted them to a diet of rather soft plants, and eventually they died out. I feel there is a valuable lesson to be had there about learning to enjoy a variety of foods..."

In Real Life
Magacerops is an extinct rhino like creature and became extinct 4.1 million years ago and as a herbivore ate plants. They were very heavy weighing 3300kg