The serpent who tried to swallow Hanuman on his way to Lanka was Surasa.
During Hanuman’s flight across the ocean in the Sundara Kanda, the gods ask Surasa, the mother of serpents, to test him. She takes on a terrifying, gigantic form and blocks his path, declaring that he must enter her mouth. Hanuman first asks to continue his mission for Rama, but Surasa insists that he can pass only by satisfying her demand.
Hanuman and Surasa then engage in a contest of size. As Hanuman grows larger, Surasa expands her mouth still farther. Hanuman finally shrinks to the size of a thumb, darts into her mouth, and exits before she can close it. Surasa accepts this as fulfillment of her challenge, resumes her true form, and blesses him.
Readers often confuse Surasa with Simhika, another supernatural female figure encountered during the same journey. Simhika attacks Hanuman by seizing his shadow and is treated as a hostile demoness. Surasa’s encounter, by contrast, is fundamentally a divine test of Hanuman’s strength, intelligence, and adaptability.