Did all Rakshasas descend only from Kashyapa or from other sages as well ?
Were Vasishta Agasthya of Rakshasa origin?
What proportion of Rakshasas were evil and what part pious and neutral judging from mythological descriptions ?
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Rakshasas and Yakshas are perhaps the only two species unrelated to Maharshi Kashyapa. They are children of Lord Brahma. The creation of Rakshasas is described in the 4th chapter of the Uttara Kanda of Ramayana as well as in Puranas like Srimad Bhagavatam.
In the Uttara Kanda, Rishi Agastya tells Rama the story of their creation, when Rama asked the same question as you.
Accordinglyy, Lord Brahma originally created denizens of water, who upon creation were very hungry and thirsty and asked Brahma what they must do. Brahma asked them whether they will guard the waters. The beings who said, "Rakshaamah" or 'We will protect' became Rakshasas and those who said "Yakshaamah" or 'We will worship' became Yakshas.
Then Brahma created two more beings Heti and Praheti to be the leaders of the Rakshasas. Praheti proceeded to do Tapasya, while Heti married Bhaya, the sister of Lord Yama. They had a son called Vidyutkesha, whose son was Sukeshi.
Sukeshi had three sons, Mali, Malyavan and Sumali. Mali and Malyavan were killed by Vishnu during a war between Devas and Rakshasas. Sumali wanted revenge, and therefore asked his daughter Kaikeshi to marry sage Vishrava, the son of Pulastya, who was Kashyapa's uncle.
Vishrava already had a wife name Idavida, whose son Kubera became the King of Yakshas by worshipping Brahma. Kaikeshi married Vishrava and their children were Ravana, Kumbhakarna, Vibheeshana and Surpanakha.
When Ravana became King, since he was the grandson of Pulastya, it became common belief that like Kashyapa was the father of Devas, Pulastya was the father (or grandfather) of all Rakshasas, which was the confusion Rama had.
So to answer the question, Ravana and his family were of Pulastya Gotra, and the rest of the Rakshasas didn't have any Gotra, as they were directly descended from Brahma. (You can call them Brahma Gotra or Heti Gotra maybe...)