Reimagining Rescue Riders dragons into the HTTYD movie universe
Rock spitters are close cousins with gronckles and hotburples. Just as cuddly and friendly, but instead of eating stones and melting them down for magma blasts the rock spitters - as their name implies use them as projectiles instead, being able to hurl them towards their target at great force. You will always lose a rock skipping contest to a rock spitter!
Similar to yaks the rock spitters posses four stomachs for storing their projectiles, or whatever treasure they have taken a liking to. All rock spitters are avid collectors, with collections ranging from junk to precious gems. These dragons are very popular amongst vikings who don't believe treasure chests are safe enough to store their valuables in. If trained correctly, then rock spitters will indeed provide great protection against potential thieves and looters. Their skin is stretchy yet thick making it almost impossible to impenetrate with any man-made tools.
Because they cannot produce fire on their own, rock spitters tend to live in deep underground caves where the temperatures are much higher. Their front claws are shaped similar to moles' and they move around by digging out tunnels. All the excess stones are stored, then used to build nests and forts. Since their "fire"power is pure physical strength, they are completely immune to the effects of the power changing crystals.
(In the show there are crystals that can weaken, or grant powers to dragons. So I came up with the theory that the hidden world is made from magical gems and the creatures originating from THW too have a small amount of magic in them, thats why the dragons in HTTYD are so different from all the other surface creatures)