Deeeep.io

Deeeep.io is an .io game created by a developer named Federico Mouse. You start out as a fish, a weak ass thing without fins to eventually become an apex. With multiple apexes in the game, there's lots of opportunities to play and try out new things, as well as many creatures to get to tier ten, or apex. There are also currently 5 gamemodes, FFA, Original, Pearl Defense, Toxic Algae, and 1v1 matches to play. Unfortunately, previously-said developer is not very active, and many people who try to reach out to him have gotten no reply. Either he's 6 feet under, quit the game, or is super sneaky to hopefully get in a Christmas update.