。
This guy was a bad rabbit.
Literal
This-guy [topic-は] bad rabbit [copula-past-だった].
こいつ is a rough, sometimes rude pronoun for 'this one/this guy' — formed from こ (this) + やつ (guy/dude). Used for animals, objects, or people the speaker has familiarity or contempt with. The katakana ウサギ is the typical writing for the animal (the kanji 兎 exists but is rare in modern text).