[EDIT: I forgot to add a content warning, I talk about Hiccup losing his leg here, so yeah nothing graphic but just in case]
Okey so hear me out, Toothless didn't bite off Hiccup's leg to save him, and I have very good reasons why I think he didn't.
First, it has never been specified how he actually lost his leg (I have however seen a video of I think it was the director? Saying that the club tail of the red death probably shattered his bones when they hit it, but take this info with a grain of salt). However most people theorize Toothless bit it off to save him from the fire. However I don't think he did that, and here are my reasons
He was wrapped into Toothless' wings. Now you may say 'well he could've bitten his leg and used that grab to pull him into his wings' but! Physics don't really work like that. They were falling from maybe 1500 meters high (clouds go from 2000-7000m, but storm ones start at around 3000-ish meters, and they descended halfway from the clouds to the ground and then turned around to shoot the red death in the mouth, and then they went back up, so I'll round it up to 1500m), and there's a thing called gravity that's a greater force than a dragon. And I know that officially Hiccup weighted 90lb in that movie, which is 41kg (roughly)(also that's like super underweight for a 14-15 yo teenager), so he wasn't like super heavy and Toothless could've easily pulled him up, but like he didn't really have the angle? I would have to show you to like, really get my point across, but imagine this: you drop a stick from super high and then jump after it and try to grab it with your mouth. Your neck and mouth do not have the necessary force or muscle to both grab the stick and pull it up so you can hug it while free falling. You'll literally hit the ground first. It's not like dragons have hands. Horribly explained, but hopefully people you can get it.
Gobber's reaction. Yes it's a "kid's movie" you can't have a character actively bleeding out. But if Toothless had actually bitten Hiccup's leg off then he'd be actively bleeding out. Which, if he had, I don't think the creators would have had Gobber telling a joke. I understand they couldn't have him saying hey if we don't treat his leg rn he's gonna die, but I don't think they'd have made that joke if he had been bleeding out, they'd have him say something like we need a healer rn. Which would be maybe a little ooc for Gobber, but blood loss can take a life in minutes literally. Also Toothless waited till Stoick apologized to reveal Hiccup. If Hiccup's leg had been torn off he wouldn't have survived, bc it's not only the moment it took Stoick to apologize, it's also however long he spent searching in the ashes. And we don't know how long it took the fire to go out to actually be able to get to Toothless and Hiccup.
People argue that Hiccup saying "but he got me back, you couldn't save all of me" in the second movie means Toothless bit his leg off, but that's not exactly true. He says you couldn't save all of me, which could be interpreted as "you missed my leg when you were trying to save me from the fire and the fall". The 'got me back' part doesn't have to necessarily be about Toothless bitting the leg off, specially followed by 'you couldn't save all of me'- it could perfectly be what I said earlier (couldn't save the leg), but made as a lightweight joke (and remember Hiccup felt guilty about being the one to rip his tailfin off, so even after Toothless forgave him, he still talked as if he deserved loosing his leg, he deserved not being saved wholly by him). Idk if I'm explaining myself right? I really can't think of words rn
Toothless has retractable teeth. Yeah I know he was desperately reaching for Hiccup, but he can retract them in ms, and he does so to shoot (that's one of the reasons he actually has retractable teeth), so I see no reason why even when panicking he couldn't have retracted his teeth. If he does so for shooting, which can be applied in a flight or fight stressful panicked situation, then he can retract his teeth to grab Hiccup.
All that said, now what I think that actually happened: his leg was burnt /shattered. Reminder, they hit the green death tail, the force of the impact could've easily shattered his bones, at least his ankle or foot (bc the amputation was below the knee I doubt it was the leg (don't actually know how those two bones are called in English)).
Also they were engulfed in flames, while Toothless was grabbing him his leg could've easily caught on fire. And fire can cauterize wounds, so he wouldn't have been bleeding out.
And whatever was left of the pedal and tail pedals gear, which was bent and broken with the impact of the fall could've also crushed his leg/stabbed it (and in that case the bleeding wouldn't been great till they pulled the rods out).
So yeah, there are lots of ways he could've lost his leg without Toothless bitting it off.
(Also, reminder that maybe even if the damage wasn't that great, in the modern world they could've saved the leg, but at that time they didn't stand a chance. Hell even if he had only slightly shattered his bones they had no actual way to reset it, and therefore he would've lost all use of his feet, making it impossible to walk. Shattered bones are set with surgery, which they didn't have the technology or knowledge to do.)
My favorite version of how he lost it, it's in @rosetta46 fanfic "run past the rivers run past all the light", where his leg is burnt and mangled, irremediable, to the point the only solution was to cut it off.
So yeah. Badly explained, but that's why I don't believe Toothless bit his leg off, and he lost it in another way
