The Serpentine is the main road in Loramendi, the chimaera capital, and it is where a huge dance takes place. It's like one of those exciting elaborate dances from Pride and Prejudice, except that no one wants to dance with Madrigal.
They stay away from her because she is assumed to be Thiago's.
Did we mention this is a costume party? People of beast aspect wear human masks, and people of human aspect wear beast masks. Fun Fun.
This costuming becomes important when a man in a horse-head mask actually has the guts to dance with Madrigal. He's very seductive and secretive. "I know who you are. I came here for you" (51.23), he says.
They flirt their way through the dance, she all sugar and sweet and he all "spice and salt" (51.36). Yum.
As the dance starts to wind down, Madrigal presses him harder to tell her who he really is.
"I came to thank you […] for saving my life" (51.40), he says.
Well, Madrigal's a freaking hero. She's saved tons of lives.
So who's this pony boy?
She leans in and looks into his mask.
When she sees his blazing eyes, she knows exactly who it is.