Smart@ss Cyborg by Amanda Milo

Old West? Check.
Aliens? Check.
Mermen…? Check.

If you’re looking for all that and a donkey sidekick, this is the book for you.
Meet William Frederick Cody, a recently liberated merman-turned-cyborg who sets off in the semi-arid (okay, desert) climate of Traxia seeking the trusty horse he’s ordered.
Instead, he’s saddled with a mischievous donkey.
The donkey unseats him at the first opportunity, forcing this merman to travel on foot through the desert under the burning alien sun.
They stumble upon Becky, a human woman who’s in the middle of having all her dreams shattered before her eyes.
William and Paco ride (…figuratively speaking) to her rescue, where William admires her very fine horse.
Once she’s safe, William makes to leave her homestead when Becky shocks him by making him a wild offer. If he protects her and her unborn child, she’ll cook his meals, clean the shanty he’ll own, and do his laundry.
She also alludes to some acts she will perform… but he’s not certain what she means. What he does know is that he’ll get to own Becky’s horse.
All he has to do is marry her.

***Note: This is a 47K story with a Happily Ever After ending and zero cheating and a really adorable donkey.