Kelpie

The Kelpie is one of those dangerous creatures of murky water. Like the nixie and kappa, they're creatures based on actual myth, a warning to others to avoid certain trecharous ponds, rivers and swamps. This creature is from Celtic mythology and, unlike the other two spirits, is wholly malicious. Its normal form is that of a pale horse, dirty with pond scum. It charms people with magic or with an attrative humanoid form to go into the water with it, then drowns them. They might even pose as someone pretending to drown, taking advantage of the good will of others.

A favourite creature of mine, not just of D&D, but of mythology in general. I read a story about a Scottish that put a bridle on a kelpie, which bound it to his will, and forced it to build a castle for him. When it was done, he let the kelpie go. Which was a dumb idea, because the second it was let go, the kelpie cursed him his land to have bad luck. There's a campaign in that story.

D&D doesn't give the creature a bestow curse spell, but it can drive people crazy with feelings of overwhelming sadness, so that's kind of a curse.