Is "slon to (jest) zwierzat" horribly wrong?
'Słoń to zwierzę' (nom) or 'Słoń jest zwierzęciem' (ins)
Słoń to (jest) zwierzę or Słoń jest zwierzęciem are correct.
I would have said that "słoń to jest zwierzę" was correct but it is marked wrong?
It's correct but somewhat strange. If you use the "to" construction, "jest" is totally redundant and rather unusual.
Added here, but there's quite a chance that you will be again marked wrong if you try it in another sentence.