"They are not small animals."
Translation:To nejsou malá zvířata.
It's simpler than that explanation. "They" is a pronoun, treated as a noun, and so are "that/those" and "it," so to is the noun in this example..
To nejsou malá zvířata. "They are not small animals. Those are not small animals." Or, To není malé zvíře. "That is not a small animal. It is not a small animal."
When the noun determiner "that/those" is used, it needs to match the gender, case, and number of its noun.
To zvíře není malé. "That animal is not small."
Ta zvířata nejsou malá. "Those animals are not small."
It is a set rule in Czech, I know it doesn't make sense but 'ta nejsou malá zvířata' sounds unnatural. Czech simply uses a singular for similar sentences. For example, 'There are five boys on the playground' would be translated as 'Na hřišti JE pět kluků', even though 'je' is singular.