Mythical creature doesn’t have one on the other hand (3)
I believe the answer is:
yet
'on the other hand' is the definition.
(thesaurus)
'mythical creature doesn't have one' is the wordplay.
'mythical creature' becomes 'yeti' (yeti is a kind of mythical creature).
'doesn't have' is a deletion indicator.
'one' becomes 'i' (Roman numeral).
'yeti' with 'i' removed is 'YET'.
(Other definitions for yet that I've seen before include "Thus far" , "Still; however" , "Up to the present time" , "'But, however (3)'" , "Still; nevertheless" .)