"This is a new animal."
Translation:To jest nowe zwierzę.
Jest by itself requires Instrumental. BUT 'to jest' is more complicated. If to is used to point to something i.e. 'to jest krzesło' you use Nom. 'To jest krzesłem' means that 'to' is some kind of object you then describe to be a chair.
Another issue is that translation from English is word by word. In everyday speech you would hear 'to nowe zwierzę'. But translating 'to nowe zwierzę' to English could be done as 'this is a new animal' or 'this new animal'. First meaning A is B, second A with some characteristics (this new). I think creators wanted to be less ambiguous.
Bottom line: 'To' with 'jest' can be tricky. Sometimes it is better to drop 'jest' - it is more natural and easier (only uses Nominative case).
Added now. Although I'd recommend not omitting "jest" if in the next part of the sentence you have a noun phrase that is neuter.
After all, "To duży tygrys" is clearly "This is a big tiger", but "To nowe zwierzę" can mean both "This is a new animal" and "This new animal". Only the dot at the end would tell us that it's the first of those options.