The screenplay is unclear, but in context it's pretty clear that it's the Erumpent under the water. Note her glowing horn, the same adjective used earlier.
SCENE 49: EXT. CENTRAL PARK—NIGHT
A large frozen pond in the middle of Central Park. Children ice-skate. A boy takes a tumble. A girl comes to help him up, they link hands.
As they are about to stand, a light becomes visible underneath the ice. A deep rumbling sound echoes. The children stare as a glowing beast glides under the ice beneath them, and off into the distance.
SCENE 54: EXT. CENTRAL PARK ZOO—NIGHT
He begins to perform a “mating ritual”—a series of grunts, wiggles, rolls, and groans—to gain the Erumpent’s attention.
Finally the Erumpent turns away from the hippo—she is interested in Newt. They face each other, circle round, undulating weirdly. The Erumpent’s demeanor is puppy-like, her horn glowing orange.
