Can I react to Rengar Pride Stalker legend trigger?

Yes, you can react to it. Rengar, Pridestalker's legend ability — "When you play a unit, give a unit +1 [S] this turn" — is a triggered ability that uses the chain (Rule 383.3). It goes on the chain as a pending item, and both players may respond with Reaction-speed cards or abilities before it resolves (FAQ #10719).