Mars is the tropical fish pet previously belonging to Sapnap, and is currently with Sapnap.
Mars was one of the first pets on the server, along with Beckerson, who belongs to Dream and George. She was named after the Twitter user @geostwt, who had donated asking for a fish to be named after them. She was originally kept in the basement of Sapnap's house, and was safe there until the conflicts of the Pet War and the Disc War.
The Pet War
After Sapnap killed Niki's fox, Fungi, she wanted to seek revenge for his death, and enlisted the help of TommyInnit. The two broke into Sapnap's house, and broke the glass windows in his basement, stealing his two fish, Beckerson and Mars, as penance.
The Disc War
When Tommy stole the fish, he hadn't realized that Beckerson, one of the fish, was important to Dream, who at the time, was in possession of his discs. Tommy attempted to get his disc back in a trade of the fish, but Dream wasn't willing at first. Eventually, the two came to an agreement in their trades, and both Beckerson and Mars were returned to Dream for safekeeping.
The War of the Burning Eiffel Tower
Towards the end of the first conflict of the War of the Burning Eiffel Tower, Dream gifted Tommy Mars as leverage over Sapnap, which he currently has in his ender chest. However, it appears that Sapnap may be willing to sacrifice Mars in order to keep a semblance of power over those he's currently working against.
Release Into the Ocean
On November 15, 2020, after a duel between Tommy and Sapnap, Sapnap agreed to release Mars into the ocean to end all pet conflicts between them. Mars now roams freely in the ocean near Rutabagville.
Right before the war, Tommy went and found Mars in the same place where she was first released, in an effort to win Sapnap's allyship.