Trilobite

The Trilobite is a standalone fossil in the introduced in  that appears in all subsequent games to date.

In
When donating to Blathers in, he will provide the following information about the fossil:

"Well, what do we have hoo? A trilobite, if I'm not terribly mistaken. Yes, yes. Very nice indeed. This is quite a beautiful specimen, as well. Quite! You're to be congratulated! Well then, where to begin? Trilobites...Ah, yes...Trilobites were hard-shelled segmented arthropods, which populated the Paleozoic seas long before dinosaurs existed. There were over 15,000 species of known trilobites, and more are discovered each year, wot wot! This makes them the single most diverse group of extinct organisms ever! Amazing! Truly remarkable, you know! They are, however, extinct: no living descendants whatsoever. The constant perils of nature and quite formidable indeed."

In
When donating to Blathers in, he will provide the following information about the fossil:

"Hoo my, simply marvelous! This fossil is an outstanding specimen. Trilobites existed long before the dinosaurs, I'll have you know. At the time, there were very few enemies, so they absolutely thrived, wot! For protection, much like a...blech...pill bug, they would roll up into a ball. Oh, forgive my babbling! The very thought of these beasts gets me atwitter!"

In
When donating to Blathers in, he will provide the following information about the fossil:

"Hoo, this fossil is a truly outstanding specimen! Trilobites were around long before the age of the dinosaurs, eh wot? They were quite numerous, and as a result, much research has been done on these fellows. In point of fact, scientists date geological strata based on them. It's almost like they're your guides for time traveling!"

In
After donating the fossil in, its plaque in the museum will read:

"Similar in appearance to a wood louse, the trilobite was an ancient sea-dwelling arthropod. It could range between 4 mm and 70 cm in length, depending on various factors. The trilobite was one of the first creatures to have eyes and the ability to detect enemies and prey."

Real-world information
Trilobites were one of the earliest groups of arthropods, and one of the most successful groups of early animals. They first appear 521 million years ago in the Cambrian period, already highly diverse. Their biodiversity, combined with wide geographic ranges and easily fossilized exoskeletons, lend to an extensive fossil record, with over 20,000 species in ten orders described over their almost 300 million years in existence. They suffered a slow decline through the upper Devonian; only four families from the order Proetida survived the Devonian period, and by the end of the Permian, only four genera from two families (Brachymetopidae and Phillipsiidae) remained. These last two families went extinct during the Permian-Triassic extinction 252 million years ago, which was the largest mass extinction in the history of Earth, killing 96% of marine species and 70% of terrestrial vertebrate species.