(Also, see the link 'shikhandi'.)
( The S in this name is pronounced like 's' in 'sip', TH like hard 'th' in 'thought' and N like 'n' in 'turn'. In the alternative spelling, the last N is pronounced like like 'n' in 'turn'. )
Sthoona or Sthoonakarna was a yaksha. He lived in a lonely forest near Drupada's capital Kaampilyam. Drupada's daughter Shikhandi was proclaimed as a son. Drupada performed her marriage with Dashaarna king Hemavarma's daughter. Later Hemavarma learnt through his daughter that Shikhandi was a woman and rushed to Drupada's place to punksh him. Shikhandi was sad about the trouble that cast on her parents. She decided to die and went into the lonely forest nearby. But luckily, this Yaksha saw herand asked her about her trouble. And as a matter of pity, he offered to exchange his gender with shikhandi for some time, until the Hemavarma trouble was over and then they should again exchange their genders to become as earlier. Shikhandi did accordingly and returned home a male. Everybody was happy and Hemavarma went back. Shikhandi then wet to Sthoona to reexchange his gender, as was agreed upon.
But, meanwhile, before Shikhandi returned, the yaksha Lord Kubera paid a visit to Sthoona's place and understood what he did. Becoming angry with him for disgracing his yaksha-hood by exchanging it with a mortal girl, he cursed his follower to remain a woman for ever. Upon being prayed for mercy, Kubera conceded that Sthoona would become a male again as soon as Shikhandi died. Thus Shikhandi remained a man for the rest of his life.
Though Sthoona was a minor role in the epic, his part is very crucial, since it was his exchanger of gender with Shikhandi that lead to the fall of great Bheeshma.