Glashtyn (Manx: glashtin, glashan, glaistyn, glastyn; pronunciation: ˈɡlæʃtɪn (?) ˈɡlæʃtən(?) 'Glosh-teen'), is a legendary creature from Manx folklore.
(etymology: The name or word glashtin is thought to derive from Celtic or Old Irish: glais, glaise, glasmeaning "stream", or sometimes even the "sea".

By some accounts the glashtin is a goblin that appears out of its aquatic habitat, to come in contact with the island folk. But others describe it as a water-horse. There was actually never a consensus in the old collected folklore about this.

But the two conflicting accounts can be reconciled by the trick of regarding the Manx glashtin as a shape-shifter. Recent literature embracing this notion claims that the equine glashtin assumes human form at times, but betrays his identity when he fails to conceal his ears which are pointed like a horse's. One modern(?) tale relates how a fisherman's daughter outwitted the glashtyn whom she recognized by his horse's ears, resisting his temptation of a strand of pearls dangled in front of her, and holding out till the red cockerel crowed to announce (prematurely) the break of dawn (Matthews & Matthews 2006,). Here it is said that the glashtyn can transform whenever upon a dunghill.