Hayagriva

A horse-head demon that Vishnu killed.
Vishnu himself also took the form of a being with a horse’s head.

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http://hindumythologyforgennext.blogspot.com/2011/11/matsya-avatar.html#comment-form

http://hindumythologybynarin.blogspot.com/2012/11/story-lord-hayagriva-less-known-form-of.html