Shinta said it, but that's exactly what kelpies do in all myths about them. Encourage a rider to think getting on is a good idea, immediately drown them and eat them. We saw one do the same in the Ancient Magus' Bride.
The encounter says more about Senshi's trusting nature than anything else. Laios knew what it would do. Senshi's experiences living in the dungeon for so long by himself have let him relax his guard or even find some beneficial.
Laios in contrast, LOVES monsters, but he also knows their behavior like the back of his hand, and knows exactly how far to trust their behavior. Shown, not told, like with the siren-mermaids.
Real growth for Senshi this episode. He relaxed his superstitions about Marcille's magics and changed his opinion after her gesture of apology. She didn't criticize him after seeing how shocked he was about what happened. She reached out in trust instead.
All this stuff is what makes the series so good.