Water Kelpie

Fey Resistance. The Kelpie has advantage on saving throws against illusion magic and is immune to charm effects. Magic cannot put the Kelpie to sleep. 

Fey Charm. When in humanoid form, the Kelpie has advantage on Charisma checks and Charisma saving throws. 

Charge. When in horse form and using at least 20 ft of either of its swimming or its walking speed at once to move towards a target, the Kelpie can try to charge at it. The target has to succeed on a DC 13 Constitution saving throw or be knocked prone.

Multiattack. The Kelpie makes two attack with its hooves.

Hooves. Natural Weapon Attack: +4 to hit, reach 10 ft., 1 target. Hit: 7 (2d6 + 2) bludgeoning damage. 

Change Shape. Action. The Kelpie transform into a female humanoid (elf, half-elf or human), or back into its horse form. It retains its hitpoints, ability scores and armor class when transformed, but gains advantage on Charisma checks and saving throws, and the ability to speak. When in humanoid form, it cannot use it's Hooves action.

Charm. Action. The target has to succeed on a DC 12 Wisdom saving throw or be charmed by the Kelpie for 1 round. When the Kelpie is in humanoid form, saving throws against this action have disadvantage.  

Undercurrent. Action. When in water, the Kelpie can create a dangerous undercurrent to push all targets within 5 ft of it away. Everyone who is affected has to succeed on a DC 11 Dexterity saving throw or take 9 (2d8 + 1) cold damage.