Thvashta was one of the prajaapathis. Once he was angry with Indra for killing his son Vishvaroopa( or Thrishiras) and created a gigantic demon Vrithra to kill Indra. Vrithra defeated Indra but Indra managed to kill Vrithra somehow, with Vishnu's help.
Prajaapathi is the name of a position held by Brahma's sons. Prajaapathi means 'one who creates'.
They were ten in number : 1.Marechi, 2.Athri, 3.Angiras, 4.Pulasthya, 5.Pulaha, 6.Krathu, 7.Vijaya, 8.Prachethas, 9.Vasishtha, 9.Bhrigu, 10. Naarada (some say Thvashta or Thvashtri here).
However, their number and names appear variously. Many other were quoted as prajaapathis. For example, Thvashta or Thvashtri was also a prajaapathi. Some consider Thvashta as the tenth one in stead of Naarada.
According to our Puraanams, the creator Brahma wanted to expand the creation in many ways. He created ten sons from his body and entrusted to them the responsibility of creation and expansion of the universes.
But, Naarada preferred penance and he took to it. The other nine prajaapathia created the various living beings and various non-living things of the universes.