When I cast Rengar Trophy Hunter on response to the Deathknell of a lonely Poro I had in defense on a battlefield, does my Rengar attack or defend?

Rengar is the Defender — your opponent remains the Attacker.

Designations are set by Contested status, not by who reacted to what: the player whose unit first applied Contested to that battlefield is the Attacker (Rule 459.2.b), and any unit played into a battlefield already Contested by the opponent enters as the Defender. Your Lonely Poro dying doesn't change that — even if it leaves you with no units there, the battlefield is still Contested by the opponent's unit, so Rengar arrives as the Defender and joins (or re-stages) combat on your side. This is the same ruling as the standard "opponent moves in, you ambush Rengar" case (RiftJudge FAQ #9992, #9820).