Sorry, but the animated movie/series Monsterous Nightmare is NOT necessarily alligator-like and isn't strictly a stereotypical dragon.
a) very specifically a dragon with a wyvern build. Which means that its front legs are also its wings.
b) Crocodile/alligator eyes are still very much side facing. A Monstrous' Nightmare's eyes are front-facing.
Notice also that the way they are slightly above the rest of the face except maybe the horns remind more of mudskipper eyes:
Though admittedly, mudskippers don't have their eyes turn into slits, the way alligators though. However, alligators aren't the only one's whose pupils narrow like that. Look at cats:
Given that cats also lent these dragons some of their mannerism — their occasionally anti-social seeming personality, their aloofness, the way they're misunderstood, a love for fish, chasing lights/shiny things, nip (except it's called dragon nip instead of catnip)... and their eyes are forward facing, and dragons hunt when it's darker out... it's a shame that instead they doggified and basic-dragonified these dragons.
Furthermore, the jaw at the middle part rounds out to look broader than it does closer to the back/top of neck, instead of with the LA where it does so at the back...
Also, I'm pretty sure the spines on the animated HTTYD nightmare are supposed to remind of flames, not necessarily of a sail. Look at the way they curve and spike out, like how someone might draw flames.
This gives it an outward representation of what kind of dragon it is: a stoker class, fire-breather that can set itself on fire.
Also, Monstours Nightmares have longer teeth than a basic dragon (or an alligator). Particularly two, almost tusk like teeth on it's bottom jaw that jut out more. It gives them personality, in a way, because it makes them look more "monstrous", sort of like some of the deep sea fish (except the top row isn't as long):
Dragons also seem to prefer eating fish (discounting the red death to which they took all the sheep and whatnot, though they do also indulge in cannibalism, I believe), for which those awkward looking long, sharp, thin teeth are better suited than the standard dragon teeth we see in western dragons and alligators.
Adult gators feed on a much bigger variety of food, which include fish, but are not limited to them. But also, their top and bottom teeth are similarly sized.
Hookfang, and Monstrous Nightmares in general only look like your stereotypical dragon design if you don't pay much attention to them.
But the western standard, basic dragon looks more like this:
(four legs, two wings, tail, simple colouration, typically red).
And Hookfang looks more like Smaug (albeit western, he has a more wyvern build):
Except he's stripy, which gives more variety to his colouration, but also lends to the fieriness, because now he looks like charred stuff and flames.
Also, it's just visually more interesting, without going overboard.
Also, he has horns like a gazelle or an impala:
Which also weirdly makes Monstrous Nightmares feel just a smidgen more like prey animals and lends itself to the whole "and we've killed thousands of them" argument.
Except, live-action monstrous nightmares don't have the pronounced, almost serrated looking rings around their horns. Instead their chunkier, bumpier... and barely look like horns.
Also, despite me using Smaug as an example, neither Smaug nor alligators have the mouth shape you want for a Monstrous Nightmare.
Additionally, I'm noticing that his feet are off.
Those claws are supposed to spread out more, which allows for better grip. Instead we get a wyvern who looks heavier, with its weight less spread out, with all of it's claws looking like that one big raptor like claw raptor dinos have, but their all dragged over the floor. Having fire, Monstrous Nightmares don't really need their claws for serious damage, but grip is useful.
Also, look at the wings of the la monstrous nightmare. See how useless those claws attached to the ends are? What's that even for? They can't use that to fight — they need those parts of their wings to be unharmed for flight — and it's not going to be useful for grip, because they with their massive jaws, they clearly prefer to attack from the front, but also even if not... how is that going to give them any better grip? They already have those front spikes for that. It just makes them look less evolved, because it gives away more clearly the fact that these used to be their forelimbs.
(Which, okay, would be fine, if this wasn't supposed to be the monstrous nightmare, but rather it's ancestor, but as far as we're aware, this isn't an ancestral dragon, but a monstrous nightmare, and, specifically, Hookfang.)
I could maybe have given a redesign a pass, if it made Hookfang's teeth look even more hook-like. But that didn't happen either.
Also, they straight up gave Hookfang Zippleback spikes.
Okay, not quite, they're thinner and longer, but...
Anyway, I don't think it's a bad design, I just think it's a bad design for the movie it's in, especially given that we're watching the movie to see these specific characters with specific traits which are also represented outwardly through their physical appearance.
The purpose of no dragon in this was to be "standard dragon". Animated movies capitalize on the privilege of not being forced to conform to expectations by creating authentic, stand out designs, and no matter how much Hookfang's colouration or Wyvern base build might imply it, not even Monstrous Nightmares are supposed to just the standard dragon designs.
Besides, standard dragon design means little in the 21st century — heck, it meant little in any other century, because dragons have always come in all kinds of shapes... even if the most common one looked kind of like the dragon on the flag of Wales.
And I say kind of, because wtf is that goblin shark looking snout and why does it have griffon ears. xD (I do love it though, really.)