That was the last straw. Your new hormones definitely weren't help you keep your temper to your rider's careless actions. You began to buck, trying to throw the mermaid off your back, unwilling to help her in whatever plan she had.
"Oof," she grunted as she landed on her plump scaly ass. Her hands quickly found their way behind her to push herself up into a sitting position, while her caudal fin flopped about on the beach, kicking up sand uselessly. You had certainly taken her by surprise, to say the least. A small part of you took a little joy from watching her struggle out of her element.
"Wha...What are you doing?! Are you crazy?," she began as she tried in vain to move her heavy tail over the sand. "I told you we'd cross that bridge when we get to it! Now, help me up, or you won't see your human form ever again!," she demanded, pushing out her bare chest as if to give herself an air of authority.
Wrong move.
You knew that she didn't have as much leverage as you did; you might be stuck as a pregnant mare, but she was stuck fast period. You began to trot towards a tidal pool, as if mocking the beached mermaid. "He...Hey! Don't just leave me here!," she shouted as you left her high and dry. Her caudal fin continued to flap, beating against the sand, while she attempted desperately to pull herself backwards and drag her tail. It was no use; her tail was just too heavy out of the water for her. Grunting, groaning, and frustrated sighing escaped from her lips during her efforts. You noticed that she was also beginning to lick her lips; she was starting to dry out and it'd only be a matter of time before she cracked and would cave to your demands.