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
Megacerops was a large herbivorous mammal that lived in North America during the Late Eocene (38-33.9 million years ago). While it resembles a rhinoceros, its closest modern relatives are actually horses. Megacerops was also far larger than any rhinoceros, reaching sizes that rival those of modern elephants. Based on the shape of its teeth, Megacerops likely preferred softer vegetation.

Males have been found with partially healed rib fractures, leading many to hypothesize that males would use their horns to fight, as no other animal known from their ecosystem could have made those wounds.