When you use 'to' or 'to jest' you put '=' between them. That's why you use Nominative.
N to N. N = N. Koń to zwierzę.
When you use 'jest' you describe subject using an object.
N jest I. Koń jest zwierzęciem.
Also using Nominative as something different than a subject is rare. Get used to using different cases. :)