From Nookipedia, the Animal Crossing wiki

An islet, or river island, is a geographical feature consisting of a small island in the middle of a river. Islets have bridges connecting them to each riverbank. However, islets are rare, and only a few towns have one. The length differs in each game. In Animal Crossing, they are tiny and are not displayed on the town's map. There is also the possibility of two of them being right next to each other. Bridges connecting them to the mainland can be either north–south or northwest–southeast. In Animal Crossing: Wild World and Animal Crossing: City Folk, they are larger than they were in previous games. In those games, every islet has a signpost for a villager's house on it, but on rare occasions the player (provided a house is generated) can even live on the island. Islets are absent in Animal Crossing: New Leaf due to the changes in the river and town's geography. They make a return in Animal Crossing: New Horizons. Due to New Horizons taking place on a deserted island, having an islet means the town is made up of two islands.

In Animal Crossing: Happy Home Designer, islets occur on the map. Due to the size of houses, they have to sit on the islet. This can be good for things like castles with moats.