Glashtyn (Manx English: glashtin, glashtan [ˈɡlaʃθən] or glashan; Manx: glashtin or glashtyn [ˈɡlaʃtʲənʲ]) is a legendary creature from Manx folklore.

The glashtin is said to be a goblin that appears out of its aquatic habitat, to come in contact with the island folk; others claim it takes the shape of a colt, or equate it to the water horse known locally as cabyll-ushtey. Yet another source claims the glashtin was a water-bull (tarroo-ushtey in Manx), half-bovine and half-equine.

Some tales or lore recount that it has pursued after women, ending in the stock motif of escape by cutting loose the skirt-hem, although in one modern version her escape is achieved by a rooster's crowing; in that tale the glashtin pretends to be a handsome man but is betrayed by his horse-ears.

In one passage, Train claims the glashtin to be a water-horse, and that this sea-glashtin would at one time emerge from his marine habitat, mingling with the local land-roving ponies, and cross breed to produce foal.

Train drew similarity to the Manx water-bull (see #taroo ushtey below) which also shared the trait of mingling with land livestock. In fact the water-bull attempts to mate with domesticated cows as well, only unsuccessfully, according to George Waldron (1731).

The glashtin, it was said, ceased to appear after the islanders started cross-breeding their native horses with breeds from the outside.

Seducer of women

• German mythographer Karl Blind noted that Manx glashtin or elashtan "attacks lonely women" as is the case with the Shetlandic nuggle and the Scottish kelpie.
• The creature was known to have great curiosity for women and pester them in rather picaresque manner, and would grab hold and tear off pieces of women's attire.

Cutting the grabbed hem off dress motif

• One anecdote concerns a glashan who caught a girl by getting a tight grip-hold of her dress. But while he slept, she cut away the dress and escaped, making him cast away the cloth, uttering something in Manx unintelligible to Campbell. Charles Roeder records a similar tale of a woman who loosened her apron-string to rid herself of the glashtin clung on her apron, and he spoke these words: 'Rumbyl, rumbyl, cha vel ayms agh yn sampyl' (The edge or skirt of the garment, I have but the sample). Sophia Morrison gives another version with this tale motif, entitled "The Buggane of the Glen Meay Waterfall".

Rooster's crowing

• In the aforementioned modern fairy tale, on a stormy night in Scarlett, the girl Kirree Quayle gave refuge to a dark, handsome stranger, but afterwards recognized him be a glashtin, deducing from his horse ears. She feared for herself knowing the creature was reputed to shape-shift into a water-horse and drag women to sea. As her fisherman father was late, she wished for dawn's break which would banish any non-mortals. She resisted his temptation of a strand of pearls dangled before her, and when grabbed she let out a scream, causing the red cockerel to crow, prematurely announcing the break of dawn, scaring the glashtin away.

Fairy fiddler

• Train also alleged that the renowned Hom Mooar (which signifies "Big Tom", a name of a fairy fiddler), was a glashtin. He goes on to supply as an example a tale taken from Waldron, describing a man was lured by invisible musicians to a strange banquet, and obtained the silver cup that came to be used for the "consecrated Wine in Kirk-Merlugh (Malew Church), even though Waldron never refers to the enchanted musicians as glashtin or "Big Tom".

Helpful spirit

• Manx lore concerning a helpful glashan was collected by Scottish folklorist J. F. Campbell, from a woman living on the Calf of Man in the southern part of the Isle of Man. The story-telling woman described a creature or being which assisted her as farmhand, performing the tasks of rounding up sheep from the fold, or threshing stalks of corn left unbundled.