Which Animal Crossing game first featured online play in the series?

Animal Crossing: Wild World was the first game in the series to feature online play.

Released for the Nintendo DS, Wild World used Nintendo Wi-Fi Connection to let players visit one another’s villages. Up to four DS systems could participate in the same village, allowing visitors to exchange items, share custom designs, and receive special gifts while connected.

The game’s online system was more structured than modern matchmaking: players generally had to register Friend Codes before visiting, and random village visits were not available. Nintendo Wi-Fi Connection was discontinued on May 20, 2014, so Wild World’s original online functions no longer operate officially.

New Leaf and New Horizons later expanded online interaction, which can make them seem like the series pioneers. City Folk also offered online play, but it followed Wild World rather than introducing the feature.