At the last moment, Fizzlepop pushed Abacus out of the way and took the full force of the blow. It was enough to fracture her horn off and send her into the river below. The creature disappeared without a trace after Cinch went into hiding.
Later on, when the coast was clear, all that Abacus Cinch could find was her sister’s broken horn. No sister at all. Devastated, Abacus Cinch took the horn back to Princess Celestia to break the bad news: Fizzlepop Berrytwist was dead.
Or was she…
It is hinted that she survived the fall and was able to pull herself from the river. But after realizing that her horn was broken, she saw herself as a disgrace to all unicorns, and vowed never to show her face in Equestria again.