How do you get the Rengar Cat Icon?
It was sold in the League of Legends store. Riot's description in the client reads: “This icon was released in the store for purchase during the 2019 April Fool's event.”
Is the Rengar Cat Icon still available?
Probably not. Riot tied the Rengar Cat Icon to 2019 (store purchase), and it does not normally bring back past event rewards or bundles.
