Spoilers for Thor: Ragnarok ahead. The first end credit scene in Thor: Ragnarok ends on a big cliffhanger involving a gigantic space ship, an ominous shadow, and an overly optimistic Thor. The scene ends with Thor and Loki on their way to Earth being confronted by an unidentified ship. Looking at the Marvel Cinematic Universe, there are three possible theories about what the ship in the Thor: Ragnarok post-credits scene is. None of these theories have been confirmed, but if you've seen the third Thor movie and are looking for answers, this is a good place to start.
Thor: Ragnarok ends with Thor, Loki, Hulk, and Valkyrie traveling to Earth on a huge space ship filled with Asgard civilians. Asgard has been destroyed in the battle with Hela, and with no other options, new King Thor decides to take his people to Earth, where he's hoping his superhero status will help them relocate. The first end-credits scene finds Loki and Thor looking out into space from the ship, debating whether or not it's a good idea for Loki to set foot back on Earth. (According to Doctor Strange, he's been declared supernatural persona non grata in the land of human beings.)
"I feel like everything is going to work out fine," Thor says before the two are interrupted. A dark shadow is cast over them, and the two look up to see what looks like a gigantic space ship. The scene ends before the ship is identified, but here are three theories as to what the ship in the Thor: Ragnarok end credits scene could be.
The most likely theory is that the ship at the end of Ragnarok is Thanos' ship. In fact, outlets including IGN and Screen Rant, as well as Redditors, all seem to agree that the ship is foreshadowing Thor's first encounter with the Avengers: Infinity War villain. Given Thor's last line of dialogue and the ominous shadow, it's clear that the ship isn't anything good, so Thanos' arrival would definitely fit the bill. Moreover, the ship, though only shown for a few seconds, does resemble Thanos' Sanctuary ship, which he uses as a base of operations, according to Comic Vine.
Plotwise, having Thanos' ship in the Ragnarok end-credits scene makes the most sense. Fans know that Thanos is on the hunt for the Infinity Stones, and it is heavily implied that Loki is in possession of the Tesseract (aka the Space Stone) towards the end of Ragnarok. It's possible that Thanos was able to track the stone, and thus find Thor and Loki, or that Loki himself made arrangements to hand it over.
This theory also fits with Marvel Studios President Kevin Feige's hints that Ragnarok would lead directly to Infinity War. "Things change drastically in Ragnarok and then build directly into Infinity War," Feige told the Toronto Sun, via /Film a few months before the film's release. Interpreted literally, this could mean that the ending of Ragnarok doubles as the beginning of Infinity War. Leaked footage of the Infinity War trailer featured Thor crash landing on the Guardians of the Galaxy ship, apparently thrown into space by some unknown conflict. Perhaps his ship was attacked by Thanos, sending him directly into the path of the Guardians?
Finally, Ragnarok producer Brad Winderbaum teased ComicBook.com that the film would compromise the cosmic universe and the "protections" Asgard might have to offer. Winderbaum could have been talking about the destruction of Asgard, but what better way to weaken the Avengers and their potential Asgard hero than by forcing an early confrontation between Thor and Thanos?
Beta Ray Bill
One theory proposed by Bleeding Cool is that the ship is actually Skuttlebutt, the sentient battleship built by Korbinites as protection. According to the theory, Skuttlebutt's brother, Beta Ray Bill (another Korbinite creation), is briefly seen on Sakaar as part of the Grandmaster's tower of champions, which means that Skuttlebutt is a part of the MCU. In the comics, Skuttlebutt attacks Thor — a plot line that could very well be picked up in Infinity War.
This theory, while interesting, doesn't seem all that likely. Marvel end-credits scenes tend to hint at future films, and the idea that Ragnarok would go without a direct link to Thanos seems unlikely.
None Of The Above
One final theory about the ship in the first Ragnarok end-credits scene is that it's nothing but a fake out. This would work with the humorous tone of the rest of the film, and be a great prank on all the MCU fans. It's entirely possible that the ship has nothing to do with future Marvel movies. It would be cruel, but not all that surprising.
Odds are we'll have to wait until Infinity War to get any solid answers on the identity of the mysterious ship. In the meantime, these theories will just have to do.