The Hera Dragon was introduced to Merge Dragons during the Owlympus Event! When you received the Golden Siren, it released 4 Hera Dragon Eggs.
You also currently have the option to purchase a Nest of Hera Dragon Eggs in the Dragon Egg Menu!
I purchased a Nest of Hera Dragon Eggs in the Live Stream below! In the video at the bottom I purchase another Next of Hera Dragon Eggs!