Guardians of the Galaxy Vol. 3 had two stories running in tandem, both of which were centered heavily around Rocket and the High Evolutionary. While the Guardians fought to save their favorite raccoon in the present, flashbacks explored Rocket's upbringing under the High Evolutionary's experimentation and torture.
Those flashbacks allowed the introduction of Rocket's animal friends - Linda Cardellini's Lylla, Asim Chaudhry's Teefs, and Mikaela Hoover's Floor. While the latter two appear to be new original creations, the talking otter Lylla has a long-standing comic bond with Rocket as an ally and love interest.
Lylla Was Referenced in 2014's Guardians of the Galaxy
Guardians of the Galaxy Vol. 3's Lylla (Linda Cardellini) was referenced in the MCU almost a decade ago in 2014's original movie, and director James Gunn likely regrets that decision.
A written profile for Rocket appears on-screen as a visual Easter egg at the Nova Corps following the Guardians' arrest. This includes a reference to him as "Subject:89P13" - just as the High Evolutionary designated him in Vol. 3.
The profile lists Lylla among Rocket's associates; however, it seems unlikely the Nova Corps could be aware of her existence given she never survived the escape from the High Evolutionary's lab.
Rocket and his animal companions became close friends during their time in the High Evolutionary's clutches. But only Rocket was able to survive their escape attempt after discovering the villain's plans to kill them as they came from an earlier, unperfected batch of his animal experiments.
Having listed Lylla alongside Groot as an "associate" of Rocket, Gunn was likely planning for the otter to enter the fray in one or both of the Guardians sequels as a love interest and ally, just as she is in the comics. But clearly, the director changed his mind later and instead integrated her into his tragic backstory.
Speaking at the E3 World Building Panel in June 2017 - via Gizmodo - Gunn spoke about the mugshot line-up scene in Guardians of the Galaxy and revealed there is a background detail in the scene he was "thinking about changing" with Vol. 3 - which he just finished his first draft for at the time:
"In the first movie there is the mugshot sequence where the characters go through and they’re going through the mugshots and having their picture taken. And the Nova Corps has information about each of the characters coming up on their machinery. Those things are something that your average viewer doesn’t see at all. But there’s a lot of background I put into those things about who they’ve worked with, where they are from, what’s going on. And it’s one of those things that I’m thinking about changing."
James Gunn's Lylla Changes Worked Out Well
On the road to Guardians of the Galaxy Vol. 3, it remained up in the air exactly how Lylla was going to fit into the puzzle, with many speculating she would be a figure from Rocket's past who he reunited with in present-day, likely as part of his mission to put an end to the High Evolutionary and his experiments.
But that ultimately did not come to pass, as Lylla never made it out of the flashbacks. Nonetheless, she remained a driving force behind the raccoon's hatred of the High Evolutionary and retroactively became a key part of his entire character arc in the Guardians trilogy.
When James Gunn referenced Lylla in Guardians of the Galaxy, it was likely just intended as a nod to the character's comic love interest and to open the door for her to become involved in his story down the line. However, Gunn probably regretted this decision once he reached the idea to integrate her into Rocket's tragic past, which clearly worked out well based on how fans are reacting to the movie.
Guardians of the Galaxy Vol. 3 is playing now in theaters worldwide.