Hero Skins were introduced on the Spring 2019 update and can be obtained by completing the Season Challenges.
Every Season Challenges will reward a unique skin to a hero.
To unlock an exclusive hero skin you must buy the Gold Tier ($4.99) for the season and complete enough challenges to get the final reward.
The first skin is the Gladiator King. Every season will reward an exclusive skin, so yes, the Queen and Warden will have special skins on the future too.
For more information about Seasons, Tiers, costs and rewards, visit our Season Challenges guide.