Can I play Rengar Pouncing to a battlefield I'm defending?

Yes, you can play Rengar, Pouncing to a battlefield you are defending.

According to his card text, Rengar has the [Reaction] keyword, which allows him to be played at reaction speed (during a Closed State, such as a combat showdown). While his card text specifically highlights an exception for playing him to a battlefield you are attacking, he also follows standard unit playing restrictions.

Under standard rules (and confirmed by RiftJudge FAQ #9876), a unit with the [Reaction] keyword can be played to your base or to a battlefield you control at any time you have the timing and resources to do so. Since you control the battlefields you are defending, you are permitted to play Rengar there as a reaction during a combat showdown.

Key Considerations:

• Timing: As a unit with [Reaction], Rengar can be played during a closed state (such as during a combat showdown).
• Location: You may play him to a battlefield you control (a battlefield you are defending) or your base.
• Attacker Status: Note that playing Rengar to a battlefield you are defending does not make him an attacker. He will not gain the benefit of his Assault 2 keyword, as that ability is conditional on him being an attacker.