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Unstable Shapeshifter (Tempest, 1997)
The reason this card exists would appear to be primarily to take advantage of a rule that no longer exists. Unstable Shapeshifter automatically changes to become a Clone of whichever creature enters the battlefield. There are certainly a number of circumstances you can imagine in order to help you exploit this card, but the primary usage was almost certainly for the purpose of hosing Legends.
What are Legends? Legends are a card type that specifies only one copy of said card can be on the board at any one time. Whereas most creatures are not specific characters, some creatures (as well as other permanents such as Artifacts, Lands, and Enchantments) are specific characters that represent figures from the storyline. For instance: you can have as many Grizzly Bears on the board as you want at any given moment (allowing for the fact that you can only have four in your deck). Conceivably, two players could have eight grizzly bears total on the table at any given moment, or create a device to generate unlimited Grizzly Bear tokens, and that would be fine.
But let's say you have a Teferi, Mage of Zhalfir on the table.
Teferi isn't a generic creature or soldier, he's a specific person who plays a specific role in one of Magic's biggest storylines. As such, he has the adjective "Legendary" on his card type line. This used to mean only one of him could be on the table at any given time, and one of the consequences of that was the "Legend rule," which stated that if two copies of the same Legend were ever on the board at the same time, both of them would go into the graveyard. (Unless you had a Mirror Gallery in play.) This is intuitive: you can't have two of the same person on the battlefield at the same time. So you can see how a card like Unstable Shapeshifter was designed specifically to dissuade your opponent from playing powerful Legends. Why cast a card if it's going to be immediately sent to the graveyard?
The problem is that for a long time Legends were not regarded with fondness by many in Magic. They were considered a creative necessity in order to be able to use the game to reflect events in a storyline, but the Legend Rule also discouraged many players from playing even powerful Legend cards because of the chances of competitive cards becoming impossible to play. (If a card was tournament-viable and also a Legend, any opponent could sideboard the same card and be prepared to nullify that exact threat in the second match. It didn't make for fun play.) Compounding the problem was the invention of Planeswalkers, a new card type created in the late 2000s to represent certain very powerful characters from the storyline (characters who could walk between different planes, or different worlds, hence the name). Although they didn't call it the Legend rule when applied to Planeswalkers, they still operated under the same principle: no more than one version of Jace or Ajani could be on the board at any given time, for the simple reason that no more than one version of the same person can be in the same room as another version.
Two things happened that necessitated a change in the rules. One, Planeswalkers became an extremely popular card type, and a vanguard of the majority of tournament-viable decks. But the Legend weakness still meant that even the most powerful Planeswalkers could be countered simply by playing a version of the same character. Two, while Legends had never been the most popular card type, that changed with the invention of the Commander format (previously known as "Elder Dragon Highlander," or EDH). Without going into detail, Commander is a format built around Legends as a card type. The rise of Commander as a popular format meant there was a great deal of new demand for Wizards to produce more Legends, even though the restrictions of the Legend rule as it existed had historically made Legends unpopular with many players and designers.
So the rule was changed a couple years back. You can still only have one version of a Legend or Planeswalker on the table at any given time, but your Legends do not effect those of your opponent, meaning you can't be forced to put a Legend in the graveyard because your opponent was clever enough to have the same Legend in their deck. And if you do have two versions of the same Legend on your side of the table, both cards do not immediately go into the graveyard, only one. All of this means that Unstable Shapeshifter can't do one of the things it was designed to do. It can still do a number of other things - there are plenty of good non-Legendary creatures still hanging around the game that you might want to duplicate, after all. But at the time of its invention the card functioned as effective removal against a certain type of card, and that functionality doesn't exists anymore - unless you're stupid enough to play this before you play a Legend yourself, in which case this copy would just go into the graveyard. And while there may be one or two exotic circumstances when that might be useful, it's just not something that comes up very often.