Hold my Reins by TJ Nichols

Can a human man capture a kelpie’s heart?

Assassin Lynck Whitehorse fled the monster realm and has built himself a good life in the human realm, teaching violin and serving coffees. He’s used to the stares and the comments about his ears and hooves, and the hook-ups that only want to experiment, but going home is impossible because of the chain around his neck. If he steps through the portal, the monster who captured him and took him away from his herd will once again control his life, forcing him to kill, and he won’t get the chance to escape again.
Since his mother’s death Gideon ‘Rox’ Roxburgh has been adrift. He quit his job as a mechanic and has been traveling, seeking a place to call home. After six months on the road, he arrives in Elder Ridge, one of the monster portal towns, to work in the auto repair shop. While in a monster town, he takes a chance and downloads the app, not expecting any of the monsters to be interested in him. Especially not the tall and impossibly pretty kelpie he matches with…
But the closer they become, the more secrets are revealed, and it’s only a matter of time before the past catches up with Lynck and Rox finds his world, and heart, ripped apart again.