How do you get the Reindeer Poro Icon?
It was a mission or event reward. Riot's description in the client reads: “Earned by completing missions during the 2017 Snowdown event.”
Is the Reindeer Poro Icon still available?
No. Riot tied the Reindeer Poro Icon to 2017 (mission reward), and it does not normally bring back past event rewards or bundles.
