Surasa is a female goddess, and she is considered as the mother of snakes. She appears in Ramayana, and she tested Lord Hanuman, while he was on his way to Lanka in search of Mata Sita.
According to legend, Surasa is one of the daughters of Lord Daksha, the mind born son of Lord Brahma, and she was married to Rishi Kashyapa, and gave birth to Holy Nagas, such as Vasuki, Takshaka and Airavata, and also she has daughters, and they are considered as the divine snake deities, they occupies the position of the demigods, and they still lives in the underworld, also called as Pathala Loka.
She was mentioned in the Puranas, and she is described as a young and charming lady, who contains an attractive face, and shines like the Sun. Surasa has helped Lord Shiva in killing of powerful demons. During the Treta Yuga, when Hanuman left Kishkinda, and flown above the ocean in search for Mata Sita, the demigods, wanted to test the powers of Hanuman, and hence they asked Surasa, to appear before him in the form of a powerful demoness.
She also appeared before him as a terrible demoness, and asked him to enter in her mouth. Lord Hanuman had agreed, and began to grow his size, and due to that, she has also expanded her mouth, then Hanuman took a small form, entered into her mouth, went inside her stomach, came back from her body, and regained his original form. On seeing his clever act, Surasa and the demi gods were very much pleased with him, and they blessed Hanuman to meet Sita Mata easily. Hanuman worshipped Mata Surasa and started his journey to Lanka.