The Elven Star
Also commonly called the 'fairy star', this symbol is used to represent all otherkin. This has roots in Pagan culture, which is also the roots of otherkinity, on top of elfkin being the first type of otherkin that we know of. You can find an essay on the meaning of this symbol here.
Represents all therians. Originally designed in 2003 in the Wereness forums. This symbol interlaces two Greek letters; the theta, meant to represent the human element and/or the soul, and the triangular delta, to represent the animal element and change.
The Diamond Heart
Represents otherhearted people. It was designed in 2017 on tumblr, and the design was settled on via a poll. The diamond symbolizes the 'other' in their hearts.
The Fictionkin Symbol
Represents fictionkin. It was designed in 2015 on the now-defunct fictionkin.com forums, and the design was settled on via a poll.