The Marvel Cinematic Universe's Guardians of the Galaxy formed in 2014 when Star-Lord, Gamora, Drax, Rocket and Groot banded together, and in Guardians of the Galaxy Vol. 2, the team's membership expanded, although Mantis was the only one to become a permanent member. Unlike her comic book counterpart, Pom Klementieff's Mantis is a full alien, though her appearance is quite tame compared to some of the other aliens we've seen in this universe. That said, originally Mantis was supposed to look a little more unusual in Guardians of the Galaxy Vol. 2, as you'll see below.
As Andy Park, Marvel Studios' Director of Visual Development, noted above, this wasn't just any piece of concept art put together during Guardians of the Galaxy Vol. 2's early stages. This was her approved design, but somewhere along the way, it was decided to tone down some of the insect-like qualities of her appearance. This version of Mantis also looks more like a warrior ready for battle, whereas the Mantis we know in the MCU is more timid and uses her empathic abilities out in the field. Still, this is an impressive design, so maybe it could be used for another member of Mantis' insectoid species should one appear in a future MCU movie.
Mantis' MCU background differs heavily from that of her comic book counterpart, as in this continuity, she was taken at a young age by Ego, and with no one else to interact with, she grew to be naive and socially awkward. Ego had Mantis use her empathic abilities to put him to sleep when necessary, but once Ego tracked down Star-Lord (his son) and the rest of the Guardians of the Galaxy, Mantis knew it was time to leave her master's side. She alerted the heroes about the Expansion and how Ego killed all of his children. Once Ego was dead, Mantis was welcomed as the Guardians' newest members, staying with them until the events of Avengers: Infinity War. Unfortunately, all the Guardians except for Rocket and Nebula faded away post-Thanos snap, but with Avengers 4 less than a year away, it's a good bet that those who disappeared (well, the heroes at least) will have their fates undone. After that, though, it's unclear what's in store for Mantis and her space-faring allies, as Guardians of the Galaxy Vol. 3 has been indefinitely delayed in the wake of the James Gunn drama.
The MCU will continue next year with the release of Captain Marvel on March 8, 2019, and Avengers 4 will conclude Phase 3 on May 3. to find out what else the superhero franchise has in development, look through our Marvel movies guide.