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Just for reference, I thought I'd point out what all Shantae can already transform into, so everyone will know what doesn't need to be included in these options, and I'm going to assume that Shantae can use any of the transformations she's ever learned already.
What Shantae could already transform into (and mostly likely still can in any other parts of Witch's Mansion where she appears):
Monkey
Elephant
Spider
Harpy
Mermaid
Tinkerbat