If my Lonely Poro dies from an enemy's unit at the battlefield and I react with Rengar Trophy during Lonely Poro's Deathknell, do I still get to draw 1 from Poro?

Yes, you will still draw your card from Lonely Poro.

According to the rules and FAQs regarding Deathknell (specifically FAQ #9841), the condition of being "alone" is a historical check—it looks at the board state at the exact moment the unit died.

Why this works:

1. Historical Check: Because the "alone" condition for Lonely Poro is satisfied at the moment of death, the Deathknell trigger is successfully added to the chain.
2. LIFO Resolution: The chain resolves in Last-In, First-Out (LIFO) order. When you play Rengar, Trophy Hunter as a [Reaction] to the Deathknell trigger, Rengar is added to the chain on top of that trigger.
3. Result: When the chain resolves, Rengar, Trophy Hunter will enter the battlefield first, and then the Lonely Poro's Deathknell trigger will resolve. Since the condition for being "alone" was already met at the time of death, the arrival of Rengar after the fact does not retroactively invalidate the trigger.

Sequence of Events:

1. Death Event: Your Lonely Poro dies while alone at the battlefield. The Deathknell trigger is created and added to the chain.
2. Reaction: You play Rengar, Trophy Hunter using [Ambush] in response to the Deathknell trigger. Rengar is added to the chain.
3. Resolution:
   • Rengar, Trophy Hunter enters the battlefield.
   • Lonely Poro's Deathknell resolves, and you draw 1 card.