Castle Story 12/5: A Song of Ice and Firewolves | event information on page 1

[AnnirasSweets]

I would think it definitely had to be Greta. In events past it's always had to be the specifically named animal ie Swanson not a swan or Harvey not a Winter Mink.

EDIT:

I checked just to be sure and you definitely cannot substitute an Ice Wolf for Greta.