Dragon Tracker - Scauldron
From Dragons Riders of Berk Part 1 Disk 2
Hiccup explains how their Dragons were sick because they ate the poisonous Blue Oleander, also known as 'Dragon Flower'. And that a Scauldron's venom was the only cure because it ate said flowers. Hiccup notes Scauldrons are huge and unpredictable Tidal Class Dragons, that are fast and stealthy swimmers and can even scare the bravest Viking. He also explains about their ability to shoot boiling hot water, and how they got the venom.