The deity Mariamman is very popular in Tamil Nadu and her name being in Tamil, cannot by definition be from Puranas, Itihasas etc. How reliable are the Sanskritic origins given to Manasa, Vitthala, Ayyappan et al ?
I'll add to this if I find more examples. I knew about Santoshi Mata but thanks to @Anisha for mentioning her.
Santoshi Mata - Her foundational Purana may be a movie! By definition - there are no "scriptural" sources.
"Santoshi Mata (Hindi: संतोषी माता) or Santoshi Maa (संतोषी माँ) is a goddess in the Hindu pantheon of whose veneration throughout Hinduism had only relatively recently commenced. She is venerated as "the Mother of Happiness", the meaning of her name. Santoshi Mata is particularly worshipped by women of India and Nepal. A vrata (ritual fast) called the Santoshi Maa vrata performed by women on 16 consecutive Fridays wins the goddess' favour.
Santoshi Mata started to be worshipped as a goddess in the early 1960s. Her prayer initially spread through word of mouth, vrata-pamphlet literature, and poster art. Her vrata was gaining popularity with North Indian women. However, it was the 1975 Bollywood film Jai Santoshi Maa ("All Hail Mother Santoshi")—narrating the story of the goddess and her ardent devotee Satyavati Vyas—which propelled this then little-known "new" goddess to the heights of devotional fervour. With the rising popularity of the film, Santoshi Mata "entered" the pan-Indian Hindu pantheon and her images and shrines were incorporated in Hindu temples. The film portrayed the goddess to be the daughter of the popular Hindu god Lord Ganesha and hindu goddess Buddhi related her to the Raksha Bandhan festival."