Rengar vs Nocturne Jungle Matchup
Rengar Jungle wins 48.3% of 1,613 Low Elo games against Nocturne Jungle, +1.6 points vs expected, 2-patch window 26.19-26.20.
Researched on LoL patch 26.20. Tips are written to hold at any rank; stats come from Low Elo ranked games up to patch 26.20.
Why Rengar is favored
Rengar has an answer to Nocturne's fear with empowered Battle Roar(W), but Rengar players still warn against taking him head-on early. They instead advise invading after Nocturne uses Paranoia(R) on a lane, punishing the exposed jungle during his gank window; that map pressure may help explain Rengar's Low Elo edge.
How to play as Rengar
Avoid a straight early 1v1 with Nocturne, and hold empowered Battle Roar(W) for Unspeakable Horror(E) if he commits; Rengar guides and players identify his early duel and the fear as the main risks.
If Nocturne spends Paranoia(R) on a lane, track his position and invade his exposed camps when your lanes can support you; Nocturne players say his jungle is often vulnerable after he uses the ultimate. (Unverified: no source confirms this note.)
Empowered Battle Roar(W) removes crowd control, so Rengar can clear Nocturne's Unspeakable Horror(E) fear if he saves the cast until the fear applies.
How to play as Nocturne
Save Shroud of Darkness(W) to block Bola Strike(E); a successful block doubles your passive attack speed. Do not rely on Unspeakable Horror(E) to keep Rengar controlled while his empowered Battle Roar(W) is ready.
Shroud of Darkness(W) blocks one enemy ability and doubles Nocturne's passive attack speed when it succeeds; this makes Rengar's Bola Strike(E) risky to throw into the shield.
Sources
These notes come from mobafire.com and r/nocturnemains, checked against the champions' abilities.