When it comes to creature types in Magic: The Gathering, they can range from super common to super rare (and we're not talking card rarity).
On one hand you have humans, dragons, elves, and many others who appear in seemingly every of the game's expansions. On the other hand, there are those which only make sense in a certain setting or as an homage to something from the game's past. And then there are the ultra-rare creatures that show up only once -- and, even then, not even in actual card form.
For this one we need to go back to the 1996. Alliances, the middle set of the Ice Age block, came out. With it came a strange and new creature type: Splinter.
The creature type made its debut and (so far) only ever appearance on an enchantment called Splintering Wind. Not only is it a rare green source of direct damage, it also makes Splinter token creatures.
Since Alliances, Splintering Wind and the Splinter creature tokens it creates haven't been back. And that's not because it's on Magic's Reserved List (because it's not).
To tell you something about it and what WotC thinks of it, it isn't even listed on the Beeble Scale, more-or-less showing that not only are there no plans to bring it back, but they don't even consider it much of a viable creature type anymore, either.
And it should come as little surprise that neither the creature type nor the card that creates it were never all that popular with players. The card's 67¢ market price (according to TCGPlayer) kind of gives that fact away.
But, even after 27 years since its introduction into the game, the Splinter creature type is one that still persists today -- even if it's a creature type that's (technically) never seen print on its own.