Learn Spanish (Latin America): No quiero matar ni un animal.

While ni usually means "nor" or "neither," it translates to "not even" when emphasizing an extreme limit, especially before a number or quantity. In negative sentences, ni un or ni una is the standard way to say "not even one."

Why is it "ni un animal" and not "ningún animal"?

You could totally say No quiero matar a ningún animal (I don't want to kill any animal), but saying ni un animal (not even one animal) adds much more emotion and emphasis. It stresses that the speaker's absolute limit is exactly zero.

Why isn't there a personal "a" before "un animal"? Aren't animals living things?

We only use the personal a before animals if they are specific, familiar, and individualized (like a named pet). Because this sentence is talking about any generic, unspecified animal, we leave the a out.

Since this lesson focuses on the passive voice, how would I change this to say "Not even one animal was killed"?

You would use the analytic passive structure taught in this lesson: Ni un animal fue matado. This uses the past tense of ser (fue) combined with the past participle of matar (matado). Because animal is masculine and singular, the participle matado takes that same form.