You can get it through the codex or breed the Leo Dragon. The Leo Dragon is the basic single element zodiac type. What I mean by that is the Leo Dragon is usually the first single element zodiac sign one can get because you don't need a zodiac type dragon to breed it.
In the instructions I posted, in order to breed a Leo, you have to breed a black type dragon (Night, Neo Black, Qilin and Noirvine as examples) with a yellow type dragon (Air, Neo Air or Sunstalk as examples) and look for a 21-hour timer.