Bulbo is a deleted character concept from the 1939 film who would have been the son of the Wicked Witch of the West whom she wanted to put onto the throne of Oz. He is known to only appear in drafts by Noel Langley along with one made by Edgar Allan Woolf and Florence Ryerson who then removed him in their second draft dated July 5, 1938.
He might have had a Kansas counterpart as Walter Gulch, the son of Miss Gulch. In one version, he was the Oz counterpart of Uncle Henry.
His storyline might have featured the witch turning a man into the Cowardly Lion so that Bulbo could marry his love Princess Sylvia instead.
For context, part of the script was included in The Making of the Wizard of Oz with the witch saying to Dorothy:
"Just wait till I've settled with the Emerald City and put my Bulbo onto the throne, then I'll have a little time to devote to you my pretty!"
And to Sylvia: "We'll make him you're best man, that's what we'll do! A lion for your best man, but you'll marry Bulbo tonight and then tomorrow the Wizard of Oz will be dead and the Emerald City will be mine!"
There also is a line with the witch talking to Bulbo, but none are available of him speaking: "There my darling boy, mother'll kiss it better! Bulbo musn't cry now, he's going to be King of the Emerald City, and kings never cry!"
A similar character was to appear in the failed Disney project Rainbow Road to Oz to be played by Tommy Kirk. Later author Gregory Maguire would introduce Liir as the orphaned son of his Elphaba in The Wicked Years series.