"These people are animals."
Translation:Ci ludzie to zwierzęta.
As Wiki says (because frankly - I don't know why) about the case of the object of być verb:
The object is either in nominative or instrumental case; the former is often used when introducing oneself, and implies the object to be definite (Ja jestem Czesław, a to jest Marysia), in the latter the object is usually indefinite (Ona jest studentką "She is a student)
"Ci ludzie są zwierzęta" is grammatically wrong. If you don't use the construction with "to", but with a form of "to be", then you have to put the animals in Instrumental: "Ci ludzie są zwierzętami". Which in this specific sentence sounds quite weird, as if you meant it absolutely literally, not as an insult.
But normally, for example "Tygrys to zwierzę" and "Tygrys jest zwierzęciem" are both perfectly natural.
Mixing the two constructions by saying "X to jest Y" or "X to są Y" is rather rare, so we usually don't even accept it automatically.
P.S. In case you haven't seen it yet, check here: https://www.duolingo.com/comment/16373167