Thanatho was a saiyan who's parents were a dragon and a saiyan due to this he was a outcast on planet vegeta but the king and royal guards were surprised and congratulated him on his immense power until they realized he didn't destroy planets due to him hating the idea of killing hundreds of innocent races so the king sent him to a planet called earth and until he destroyed it he would not be able to go back this is how he survived the destruction of planet vegeta he started growing up at the time he was sent to earth right after goku as a back up plan he eventually met goku and became friends with him and helped him in fights and was a member of the Z fighters he also can transform into multiple dragon forms along with super saiyan forms he has mastered the dragon fist and knows a good deal of goku's moves and some other z fighter's moves
NOTE:ALL ABILITIES LISTED HERE ARE ONES USED AS GOING IF HE WAS AT THE END OF GT. DRAGON FORMS ARE NOT USED AT ALL TIL LONG INTO THE STORY NOR SSJ. KEEP THIS IN MIND.