The Teenaxi were characterized by their short stature, measuring about a third of the size of an average Human male. Featuring large expressive eyes and mouths, the Teenaxi had short hind legs and long arms, giving them the capability of quadrupedal locomotion.
When agitated or angered, the Teenaxi were able fighters, rolling themselves in balls and launching themselves at their target, operating in large numbers to overwhelm a much larger opponent. (Star Trek Beyond)
In the alternate reality, the Teenaxi were offered an artifact from the Fibona via Captain James T. Kirk. However, during negotiations, the suspicious leader of the Teenaxi became convinced that the Fibona intended to use the artifact as part of a plan to "eat them" and the delegation attacked Kirk in a panic, some being beamed onto the USS Enterprise by accident during his escape.
Star Trek Beyond co-writer Simon Pegg compared the Teenaxi's function in the film to the Tribbles of The Original Series, saying "The Teenaxi sequence with the tiny aliens at the beginning was the first thing we wrote, and it was kind of a joke. But when we came to the edit we were wondering whether it should start with the [action sequence of the] shuttle coming out of the nebula. But that didn’t work. It had to start with a lightness. Our initial thinking was let’s make the opening of the film a sort of lighthearted episode of the show, like “The Trouble with Tribbles.""