Can you play Rengar Pouncing at reaction speed to a battlefield that you are attacking and currently in a combat showdown for?

Yes, you can play Rengar, Pouncing to a battlefield you are attacking during a combat showdown.

While Rule 739.3.a generally restricts playing units to your base or battlefields you control, Rengar's specific card text provides an exception: "I can be played to a battlefield you're attacking." Because Rengar also possesses the [Reaction] keyword, he can be played at reaction speed during a combat showdown (a closed state).

Key Considerations:

• Timing: As a unit with [Reaction], Rengar can be played during a closed state (such as during a chain in a combat showdown).
• Location: Rengar's ability explicitly overrides the standard restriction on where units can be played, allowing him to be placed at a battlefield you are attacking, even if you do not control it.
• Attacker Status: Note that while you are attacking the battlefield, Rengar enters play without the "Attacker" designation. According to the established rulings for Rengar, he gains the Attacker designation and becomes Mighty (for effects like Assault) during the cleanup triggered by his entering the board.

This is my interpretation based on the card text of Rengar, Pouncing and the rules regarding the [Reaction] keyword. No official FAQ exists for this specific interaction beyond the general rules for Rengar's play timing and location.