When Hiccup, Toothless, and Astrid find the dragons nest, all the dragons deliver large amounts of food into the mouth of the volcano save for one Gronckle who lazily only offers one fish and is immediately eaten by the Red Death. Fishlegs is seen flying the Gronkle that was used in the training arena in the film.
The official Website describes the Gronckle as follows: "One of the toughest Dragons in our world is the GRONCKLE. Gronckles have gigantic heads, short bodies, and round tails. Gronckles are lazy, and spend most of their time sleeping in communal heaps. They have been known to fall asleep while flying, waking only when they crash into the ocean or the side of a mountain. Gronckles have relatively small wings that beat as fast as a hummingbird's. They are slow in the air, but make up for this with their maneuverability. A Gronckle can fly backwards, sideways, and even hover. Gronckles attack from the air, where their primary weapon is most effective. Gronckles chew rocks, melting them in their bellies and firing them as balls of molten lava."
"With its bulbous shape, tiny wings and laid-back demeanor, the warthoggish Gronckle is proving to be a fan favorite. Says Sanders, “He is a series of round balls, including warts. He’s just lazy, cantankerous and grumpy, and that makes you like him even better.” The critter is known to doze off even in mid-flight as he floats about like the Goodyear blimp. But there is no denying that his fire packs a lethal punch."
In the Film
The Gronckle is the first dragon that students have to fight in the arena. It is used to teach them how to use the shield to block attacks. One almost blasts Hiccup, but he is saved by Gobber the Belch. Hiccup learns from Toothless that dragons love garlic grass and uses this to rub the same Gronckle into submission in a later training session. The Gronkle is used to determine who will kill the Mosterous Nightmare and is seen subdued by Hiccup (presumably through scratching a particularly sensitive spot dragons have), earning him that right. Hiccup later gives the Gronkle to Fishlegs during the attack on the Red Death and during the battle, it hovers beside the evil dragon's head, allowing Fishlegs to distract the Red Death by banging on his shield. This also confuses the Gronkle and causes it to crash and flip upside-down, pinning Fishlegs who ends up being okay. The Gronckle and Fishlegs are nearly crushed by the Red Death's foot, but thanks to Snotlout's antics, it barley misses them and the Gronckle is later seen flying around the villiage as Fishlegs' pet dragon.
In the Book
It is shown in the book that Dogsbreath the Duhbrain and Wartihog both have Gronckles in the Pirate Training Programme. Dogsbreath's is called Seaslug. Stoick the Vast also has a Gronckle called Newtsbreath.
(Taken from How To Train Your Dragon Wiki)