Octoballs are small, round Octarians with one large eye and and a pair of purple lips. Because of their body shape, they are able to roll on smooth surfaces while leaving behind a thick trail of purple ink. When shot with Inkling ink, they are pushed backwards, but not harmed. However, if submerged in a puddle of Agent 3's ink, they will struggle to move and can be destroyed.
A total of 9 Octoballs can be found in Splat-Switch Revolution. 3 of them are located in Checkpoint 1. 6 of them are located in Checkpoint 4.
They do not appear in Octo Canyon, but sanitized versions do make an appearance in Octo Expansion, though never used as enemies, with them being trapped in storage units during the Belly Phase as well as inside the liquid lasers of the NILS Statue.
In the end of the fan-made issue of Splatoon, Specs (an Inkling from Team Blue) flies over a bunch of Octotroopers and Octoballs. It's most likely that they would not appear in the manga again. But will possibly do so due to the Inkopolis Tower not having the Great Zapfish on it.
Tips
The Octoball can be easily defeated by letting it swim in Agent 3's turf, in which the animation of the Octoball will look like it's trying to swim escaping the ink. When that animation plays, Agent 3 can easily splat the Octoball.
Trivia
Despite Cap'n Cuttlefish saying they must be submerged, Octoballs can also be defeated by knocking them out-of-bounds. This is useful for levels with the Inkling Girl amiibo levels as they can be sniped off the cliffs while in their own ink.