Had a look in Dando-Collins' "Legions of Rome". He notes 9th Hispana's emblem was the Bull and its 'birth' sign Capricorn (probably).
Just a caveat there: The bull badge is an educated guess based on the idea that (some? all?) legions founded/deployed by Julius Caesar received such an emblem. You
could use this, but you don't have to in order to depict
the Ninth.
No idea where the information on a 'birth sign' comes from (the 9th legion's origin is as obscure as its disappearance), but maybe another neat idea for decoration. Neither more nor less likely than any other motif, I'd say.
Edit: If anyone's interested, there's a fairly up-to-date
article on the Ninth on Livius.org.