The Boggart Fights Back by Susan Cooper
Susan Cooper has worked her magic again. Yes, the Boggart is back and this time he's bringing reinforcements, including Cousin Nessie of the Loch Ness. Boggarts are "Very ancient creatures, of the Wild Magic...They are shape-shifters, they can become anything they chose." The two, along with siblings Allie and Jay, find themselves at odds with William Trout, a real estate developer who plans to turn Castle Keep, the family's ancestral home, into a resort complex. He has already purchased much of the property. Bulldozers have commenced tearing down existing forests surrounding the castle.
To thwart his plan, the kids and monsters begin scheming. When ordinary pranks such as dumping a bulldozer into the Loch fail to stop Trout, it is time to call upon Scotland's mythological creatures to save Castle Keep. Note: The premise of a real estate magnate, declaring that the project will "be a huge success, huge...bringing hundreds of jobs...I'm a very smart businessman..." may sound familiar to today's readers.
A highland banshee. In this tale she is a small old woman in a green gown, found at the base of a waterfall.
A shape-shifting water horse who kills and then devours humans.
Blue Men of the Minch
Denizens of the Western Isles, possessing blue hair and long blue beards. They prey on passing ships and are responsible for fierce storms.