The story of origin of 11 Rudra and 11 Rudranis is found in the Vishnu Purana. Rudra Shiva had appeared in the form of Brahman once and this is mentioned in the Kena Upanishad, composed several hundred years before Vishnu Purana.
Seeing Adharma flourishing on earth and Mother Earth suffering, Brahman became angry. From his curved eyebrows and forehead burning with anger rose Rudra, luminous like afternoon sun. His body was huge and it was half male and half female.
Rudra then separated male and female portions of his body into two. He further divided the male portion of his body into eleven distinct parts. The female portion of the body too split into eleven distinct parts. Some of these forms were gentle, some ferocious and cruel and some fair and some black, some peaceful and some turbulent.
The eleven male entities that appeared from Rudra came to be known as eleven Rudras and the eleven female entities came to be known as Rudranis.
The names of the eleven Rudras are different in different Puranas.
The names of 11 Rudras in Vayu Purana are Ajaikapada, Ahirbudhanya, Hara, Nirita, Ishvara, Bhuvana, Angaraka, Ardhaketu, Mrityu, Sarpa and Kapalin.
The names of 11 Rudranis as per Chapter 7 of Vishnu Purana are Dhi, Vritti, Ushana, Uma, Niyuta, Sarpis, Ila, Ambika, Iravati, Sudha and Diksha.