Poseidon knew he should stop, knew he should let her lead him back to the stables and 'put him away' but he felt good, the rush of being ridden was almost too much for him as he crossed the paddock at a flat out run. He snorted and tried to resist the change in her seating and the grip in his mane but she was forceful and knew what she was doing.
He whined, his tone defiant as he slowed to a walk and came around in the direction she wanted him to go but doing it circumspectly. He had zero intentions of letting it go too far because Jeremiah was already rebelling in his head. He sauntered toward the stables, his mind filled with thoughts of changing back and taking her in the hay of a stall. Oh shut up, Jeremiah growled as he shoved at Poseidon's grip on their body. Normally he could shove hard enough to make him lose the horses form but with a rider he didn't dare risk it.
Poseidon was distracted, something about the young woman on his back was different. Poseidon reached out to her and pushed at her mind. A god soul. Poseidon cursed his distraction, he hadn't even noticed. She wasn't familiar, he didn't recognize her like he had recognized Demeter and Hera.
He stopped at the stable doors, his muscles quivering and he stomped his foot impatiently. She was a goddess or god, he could change forms without hesitation and demand to know who she was. If she would get off of his back. Torn between desire and frustration, Poseidon let out a decidedly human sounding snort.