The Boobrie haunts the sea lochs and deep inland waters of the west coast of Scotland, especially the waters of Argyllshire and the islands beyond. In its most common form it appears as a gigantic water bird, something between a cormorant and a great northern diver, but scaled to monstrous proportions. The folklorist Campbell of Islay, who collected the tales of the Highlanders in the 19th century, recorded that one authoritative source described the creature as larger than seventeen of the biggest eagles put together. Its neck is almost a yard long, its black beak measures seventeen inches from base to tip with the final five inches tapering like an eagle's, and its short powerful legs end in webbed feet armed with claws that can grip a calf and carry it to the deepest water. The creature's wings are better suited to swimming than to flight, and when it moves through the water it leaves barely a ripple, a dark shape sliding beneath the surface until it strikes.

The Boobrie does not keep to one shape. It can transform into a water horse, the Each-Uisge of Scottish folklore, a black horse that gallops across the surface of a loch as if the water were solid ground, the sound of its hooves indistinguishable from the beat of hooves on earth. In this form it has lured farmers to disaster. One tale from John Gregorson Campbell tells of a farmer and his son ploughing near Loch Frisa on the Isle of Mull. When one of their horses lost a shoe, they spotted a stray horse grazing nearby and harnessed it to the plough. Everything went well until they reached the stretch closest to the loch, where the new horse became anxious and refused to move. The farmer tapped it with his whip, and the animal let out a deafening bellow as it changed into a gigantic Boobrie. Before the men could react, it plunged into the loch, dragging the plough and the three original horses into the depths. The farmer and his son watched in horror as the surface closed over their lost team, and though they waited seven hours, nothing ever rose again.

In another form the Boobrie becomes a water bull, a tarbh uisge, a colossal black bull that bursts from the loch to interact with humans. In a tale recorded by George Henderson, a man named Eachann found a huge black bull lying close to death beside Loch nan Dobhran on the Argyll coast. He took pity on it and fed the animal until it recovered. Months later, his sweetheart Phemie was staying alone in a sheiling near the loch when her violent former lover Murdoch appeared, tied her up, and tried to abduct her. At that moment a massive water bull charged from the loch, knocked Murdoch to the ground, and knelt so that Phemie could climb onto its back. It raced with impossible speed to her mother's house, then disappeared. A voice was heard in the air, speaking in Gaelic, declaring that it had once been helped by a young man and had now aided a maid in distress after three hundred years of bondage. The tale reveals the persistence in folk belief of the idea that the Boobrie is the abode of a spirit, a creature bound by some ancient debt or curse that can only be broken by acts of kindness.

The Boobrie has a third and stranger form. During the summer months, usually in August and September, it may appear as a large striped insect, a gobhlachan or earwig with many tentacles or feelers, that sucks the blood of horses. This form is seen infrequently, and it adds to the creature's reputation as a thing of pure malice, a pest that attacks the very animals on which Highland farmers depend for survival. Whether as bird, horse, bull, or insect, the Boobrie brings loss and dread, the explanation for every calf that vanished from a boat, every horse that went lame near the water, every strange sound that echoed across the loch at night.

The name Boobrie may derive from the Gaelic boibhre, meaning cow-giver or cow-bestowing, or from tarbh-boidhre, which the Scottish lexicographer Edward Dwelly recorded as meaning "monster, demon" and "god capable of changing himself into many forms." The folklorist Campbell of Islay speculated that the creature may have grown from sightings of the great auk, the extinct flightless bird of the North Atlantic, while the bellowing cry that gives the Boobrie its bull-like voice may come from the common bittern, a secretive bird with a low echoing call that was a rare visitor to Scotland. Whether it began as a misidentified bird, a memory of an older god, or a spirit bound to the waters, the Boobrie remains one of the most terrifying creatures in Scottish folklore, the shapeshifter that reminds every traveller that the loch is not empty, and that the thing beneath the surface may be listening.