Vatnavaettir are horses made out of water. Once you touch them, you are stuck to them, dragged into the water, and drowned.
Magnus Chase and the Gods of Asgard[]
The Ship of the Dead[]
While the crew of the Big Banana docked in York, they split into two groups. Magnus Chase, Alex Fierro and Thomas Jefferson Jr. headed into the town while Samirah al-Abbas, Mallory Keen and Halfborn Gunderson guarded the ship from these creatures. Halfborn hated these creatures while Mallory knew Gaelic songs to calm these creatures and Samirah was good with horses. These factors helped them guard the ship but with a heavy payment of Mallory's Red hair bring cut to her shoulders, Halborn's shoulder slumped with a broken shoulder and Sam almost being drowned to death.
After Magnus defeated Loki in Flyting, the crew escaped using the Vatnavaettir which were actually befriended by Sam.