After fulfilling the requirements you will acquire dress parts of that fapet for Nishy to wear.
1. Tame 10 fapets of the desired type (10 bunnies, or 10 sheep, or 10 kelpies..)
2. You'll receive a quest to unlock the dress. Complete it to recive the dress parts.
3. Tame up to 20 fapets of that type to fully unlock the dress (hair style, accesories, panties).
For more details can be looked up in Book[TAB] > Help, search for 'Cross Dresses'