I have always thought that topiary animals are a bit bizarre, actually to be correct I think they are a bit creepy (especially giant rabbits for some reason)*  However, I can see how the challenge of creation would be rather appealing.  Trimming any tree or shrub is a very slow form of sculpture, what you do this year will affect the coming years and every single cut eliminates certain growth patterns permanently.  You can’t glue the branch back on.  Now in trimming an apple tree (today’s pastime) such a mistake is less a mistake and more a choice, since hopefully one is choosing the healthiest branch or simply choosing between two fairly equal branches.  The goal is less about aesthetics and more about the overall health of the tree; though I am as concerned about aesthetics as I am with the apples from this set of trees, which is part of why it takes me forever to trim them.  However, imagine making a mistake about a prospective topiary giraffe, how frustrating it would be to end up having to decapitate it a decade into the project because you choose the wrong branches for the neck!  Though I suppose you could create an okapi out of it…

*weird recollections of Monty Python perhaps, though that rabbit was normal sized?