Back in the day, we had [scryfall]Plague Rats[/scryfall]. That wasn’t a big card. Sure, everyone knew about it and probably everyone who had seen it had thought about using it, because we didn’t have the deckbuilding restrictions we have today. In 40 card deck with – say – 14 Swamps and 26 Rats (these days probably 15 or 16 lands and rest for the Rats), those could be pretty bad for you. We even had one of those decks as a gauntlet deck (a deck your deck has to be able to beat in order to be considered playable) for a while.
Over the years, they tried again. We had [scryfall]Pestilence Rats[/scryfall] which didn’t care only about others with a same name, but all rats. Then there were [scryfall]Relentless Rats[/scryfall] which was closer to the original [scryfall]Plague Rats[/scryfall], but partly by changing the rules on card limits. They are still reportedly pretty hard to get, because they are pretty popular in casual play.
Then, they finally got it right. [scryfall]Pack Rat[/scryfall] wasn’t taken very seriously year and a half ago, but over time they have become a format defining card after Kentaro Yamamoto played them in his Pro Theros monoblack devotion deck to a Top 8 finish over a year after the card first came out. Its actually such a format defining card that its beginning to lose popularity because everyone is expecting them and have weapons at the ready, including moving back to Esper.