I think a crocodile fruit could also work, but it would be a tad underwhelming.
Actually "Ryokugyu" is just his epiteth, not his real name, just like the other Admirals, so there may not exactly exist a correlation between his epiteth and his DF.
Aokiji means "Blue Pheasant", Akainu means "Red Dog", Kizaru means "Yellow Monkey", Fujitora means "Westeria Tiger", and Ryokugyu means "Green Bull".
Anyway, the old Admiral Trio have Logia DFs, Fujitora's is a Paramecia, so Ryokugyu's could really be a Mythical Zoan, and not an Ancient Zoan like the Calamities, much less a regular Zoan like Jabra's.
It could even be a Logia or Paramecia honestly, but being a Zoan makes a lot of sense in this case, the Admirals already have Logia and Paramecia.