If Rrngar plays from hand on a contested open battlefield and wins, he was made the defender. Does he get a conquer point?

Yes, Rengar does get a conquer point in that situation.

According to RiftJudge FAQ #5137, a player can score by gaining control of a battlefield they haven't scored yet that turn, regardless of whether they were the attacker or the defender. This is often referred to as "conquering on defense."

Here is the breakdown of why this happens:

1. Gaining Control: The battlefield was contested and Rengar joined as a defender. Once combat resolves and your opponent's units are defeated (leaving only your units present), you establish control of the battlefield.
2. Scoring Criteria: Per Rule 464.1, you score a Conquer point when you gain control of a battlefield you did not yet score that turn.
3. The "Attacker/Defender" Distinction: As clarified in the FAQ, the terminology of "attacker" and "defender" designates combat roles, but it does not prevent the defender from scoring the Conquer point if they end the combat in control of the battlefield.

Sequence Summary:

• You play Rengar to the contested battlefield (joining as the defender).
• Combat resolves; your units are the only ones remaining at the battlefield.
• You establish control of the battlefield.
• Since you have not yet scored this specific battlefield during this turn, you gain 1 Conquer point.

Note: You can only score each battlefield once per turn. If you had already scored this battlefield earlier in the turn, you would not gain another point.