Shape-Shifting, Galloping Celtic Sea Monster
Puzzle 20 of Building a House Hard Pack
Question is: Shape-Shifting, Galloping Celtic Sea Monster
and the answer is: KELPIE
Word Meaning:
KELPIE — (Celtic mythology) A malevolent shapeshifting spirit, most often in the form of a horse, believed to haunt the rivers and lochs of Scotland.
Definitions from Wiktionary.