After cutting, Medusa's head continued to exert power. Either the sight of it full-in-the-face or the look of the 2 eyes turned humans to stone.
Medusa - Tête de Méduse, by Rubens (c. 1618).
Public Domain. Courtesy of Wikipedia
The children of Poseidon and Medusa were born after Pegasus sliced off Medusa's head. One was the winged horse Pegasus. The brother of Pegasus was Chrysaor, king of Iberia.