Agar.io was the first.
io games ever created. It was created by Matheus Valadares in 2015, and 4chan contributed significantly to its popularity. The game centres on cells that consume agar and consume other players in order to expand. A few months after its publication, Miniclip purchased the game, and a new gaming genre centred on the humble.io domain name began to develop.
The subsequent games concentrated on the growth mechanisms that made Agar so successful, therefore establishing.io as a genre and not merely a domain extension. Slither.io was one of these games, with the same basic, but addicting gameplay, but with snakes. There are currently several grow games like Snake.io and Wormate that expand upon this premise of fast-paced survival.