My husband used to work for Fox Animation (before 
they closed) and Anastasia was all colored 
digitally... 

However, a few animators had a habit of making 
painted cels for themselves... They would 
photocopy production drawings onto acetate then 
paint them just like traditional cels, and hang 
them by their desks and around the studio for 
decoration.  So eventhough the cel you found may 
not have actually been used in the production... 
it was probably still drawn and painted by a Fox 
animator.