If you've been following Captain Marvel's promotion at all, there's a good chance you know about Goose the cat–he's easily the MCU's latest breakout star, despite being, well, at cat. He even got his very own character poster right alongside Brie Larson and Samuel L. Jackson–not bad for someone who can't actually speak and doesn't have opposable thumbs.
But the truth about Goose is actually a little weirder than what you might assume. Sure, the internet loves cats, and the MCU has been sorely lacking in plucky animal companions since Rocket was introduced back in Guardians 1, but Goose is actually a lot more than meets the eye–and way, way more than a mascot for Carol and Nick's fun buddy cop adventures through the '90s.
Here's your obligatory spoiler warning: Major Captain Marvel plot points discussed below.
The truth is Goose is no ordinary cat–he's actually not a cat at all. Like his comics counterpart, Chewie, Goose is actually an alien known as a Flerken, a funny name for a monster that is really anything but.
Introduced fairly recently in Captain Marvel #1 back in 2014, Flerkens are a sort of dangerous alien creature that look shockingly similar to Earth's cats, but are in fact deadly, monstrous eating machines that procreate by laying over a hundred eggs at a time and can run the risk of infesting any area they call home. They've got tentacles in their mouths, voracious appetites, and stomachs that contain "pocket dimensions" that allow them to consume things much larger than their bodies without issue. In a weird way, they're almost like Star Trek's infamous Tribbles–but a bit more deadly and sly.
A Flerken infestation could prove deadly if left unchecked, which is why most people who know what they are and how to spot them react pretty violently to them–or, they just try to get as far away from them as possible. It was Rocket Racoon who first realized the truth about Chewie in the comics–which Carol herself vehemently denied, until he was proven right a bit later when we saw Chewie in a nest of all his Flerkhen eggs.
The movie handles Goose similarly, with Talos filling the role of the Flerken-spotter. Our confirmation of Goose doesn't come from a nest, however, but a starring role as the heroes' catch-all secret weapon, using his tentacle mouth and interior pocket dimension to not only kill a handful of Kree soldiers, but also to spirit the Tesseract back to Earth unnoticed.
The biggest question for the MCU is how, exactly, did Goose end up on Earth in the first place? He's not given any real origin story, and instead just shows up at the Project Pegasus base as Nick and Carol work to infiltrate it. The simplest and most obvious answer would be that Mar-Vell herself brought Goose to Earth while she was undercover, but what has he been doing for all this time since Mar-Vell's death and Carol's disappearance if that's the case? Protecting the Tesseract? Sneaking food and shelter from SHIELD? Just getting really into being treated like a house cat? And why would Mar-Vell bring a Flerken to Earth to begin with?
Who really knows? It's possible we haven't seen the last of Goose–or, at the very least, of his little Flerken relatives. He was on Earth for an awfully long time unsupervised, and if he's anything like Chewie, that means there's probably a big Flerken nest somewhere hidden on the Pegasus base. Maybe they've already hatched and there are hundreds of unsuspecting humans out there unknowingly caring for eldritch aliens.
What if the Flerken are the real invading force we all thought the Skrulls would turn into? Watch out, Thanos, there might be a new big bad in town.
Probably not. But maybe. Listen, they're a cat with other dimensions in their stomachs. They can eat Infinity Stones and hack them up like hairballs. Tell me that's not at least a little bit terrifying.