Every one things that Vishnu’s avtar starts from Dasa avatara but there are four main avatar before dasa avatara.
1. Purusha avtar:
This is Vishnu’s original avtar. In that he is taking as Vasudeva, sankarshan, prathumna and as anirudha.
2. Guna avtar :
In this avtar vishnu taking as Bramha , Vishnu and Rudhra to show three gunas like Rajas, Thamas,sathvik.
3.Manvantara Avtar:
This is the story of Manu. In this Vishnu telling the Geetha to manu and then only to Arjuna in Krishna avtar.
4. Sakthaseva avtar:
Apart from there are sevsral avtar it all in the mahabharatha tatpariya nirnaya
!! Hare Srinivasa !!