- Advertised Extra: Eret son of Eret received this treatment. Despite Kit Harington received his own promo video full of Actor Allusion, Eret's role is very minor and his biggest contribution to the plot is identifying the new Big Bad Grimmel.
- Ascended Fanon: "Light Fury" has been used before (albeit rarely) to refer to a white, blue-eyed counterpart to Night Furies. This film canonizes them.
- Casting Gag: The warlord Ragnar is voiced by Icelandic actor Ólafur Darri Ólafsson, who had previously voiced Stoick the Vast in the Icelandic dubs of the previous two films. Ragnar shares a body model with Stoick.
- Channel Hop: This is DreamWorks's first film to be released by Universal after Comcast's acquisition of the studio. On that note, the first film was released by Paramount, and the second by Fox.
- Deleted Scene:
- The scene where Hiccup makes Toothless a new tail fin so he can follow the Light Fury was originally storyboarded to be a two minute-montage, but is shortened in the film proper to only last about half a minute.
- There was going to be a flashback of young Hiccup finding a wounded bird in the forest. Stoic would ask him to look away so he could end its pain, but Hiccup would plead to let him help the bird instead. Stoic would concede, but tell Hiccup it would be his responsibility to care for and heal the bird until he could release it into the wild again. The scene would thus have served as foreshadowing for the ending with Hiccup letting Toothless go.
- Another storyboarded scene had Hiccup attempt to make Toothless and the Light Fury a "love nest" in New Berk to convince Toothless to bring her back there, before Valka arrives and discusses with Hiccup what really is best for their dragons.
- Another way the film would have opened was of Hiccup and Toothless travelling to a sea cave to try and recruit a sea dragon living there, only for Hiccup to be in trouble when it attacked, leading to Toothless simply growling at it to make it yield to his kingly status.
- The scene of Toothless flying with the Light Fury originally went on for much longer and showed them playing and bonding.
- Executive Meddling: According to an Insider article, Dean DeBlois had little input in T.J. Miller being recast; the latter had recorded all his dialogue, and when Justin Rupple came in, he had to match Miller's performance to the completed animation.
- Fake Russian: American F. Murray Abraham gives Grimmel a Russian accent.
- Fan Nickname: The white female Night Fury-like dragon was given the nickname "Light Fury" by fans, although it soon turned out that this was what her species is actually called. Some have also taken to calling her "Nubless," and less charitably, "Girl Toothless."
- I Knew It!:
- Many artists and fanfiction writers had already depicted a potential love interest for Toothless as pale/albino years ago.
- Many thought that Toothless' love interest would look way more cute or distinctively feminine than Toothless, which in regards to the first official poster is exactly what happened.
- Many guessed that Hiccup would have a beard in the third film. The first trailer reveals that he does have one during at least one point in the film, either later on or as a Framing Device.
- Many predicted T.J. Miller would be replaced as the voice of Tuffnut, especially since, unlike Deadpool 2, there was enough time to replace him. Sure enough, once the early reviews came out, it turns out they did.
- The Other Darrin: When the cast was announced, T.J. Miller was meant to reprise his role as Tuffnut in the film. Justin Rupple replaced him on account of Miller's controversial behaviour in 2018. Also counts as The Other Marty in the context of this film.
- Short Run in Peru: The film was released weeks earlier ahead of its February 22 United States premiere in markets such as Australia (January 3), Norway and the Netherlands (January 11), South Korea (January 30), and Italy (January 31).
- Spoiled by the Merchandise: Despite being a guarded secret in all of the toyline, Hiccup's and Astrid's children Zephyr and Nuffink are revealed in a upcoming Nintendo Switch game in the online store.
- In a similar way, the Night/Light Fury babies are featured both in the Playmobil sets themed around the movie and also got their Funko Pop! figures, both unveiled weeks before the movie's release.
- What Could Have Been: When the film was first in production, Word of God revealed that Drago Bludvist would return from the second film and become a more developed character, possibly even performing a HeelFace Turn. However, DeBlois later revealed after the release of the first trailer that Drago's intended role was removed in the script's rewrites.
- Word of God:
- Director Dean DeBlois notes that the Light Fury is not another Night Fury, but a distinct relative of the species. Her ability to disappear through fireballs is caused by the fire heating her scales to become reflective of her surroundings.
- DeBlois revealed in a behind-the-scenes video that Hiccup and Astrid are both 30 years old in the epilogue, with nine years having passed since the dragons left.
- Word of Saint Paul: According to Cressida Cowell, Hiccup and Astrid's children are called Zephyr (girl) and Nuffink (boy), though this isn't mentioned in the film itself.
Trivia / How to Train Your Dragon: The Hidden World