Brace yourself for a long, rushed post.
Yes, it is a free, computer, online game. (I think there is a mobile version... not sure.)
DreamWorks created it, and it is highly accurate, much to the pleasure of HTTYD fans.
It is an open-world game with 60 dragon species (all found in the movie and Netflix series) to choose from. You can raise as many dragons as you want! You have a farm, stables for your dragons, quests to complete. Um... there is A LOT to the game, lets see... fishing, dragon upgrades, racing, battles, character and dragon upgrades, flight and battle training, and join a clan if you wish. (Much more, but that is a summary of a summary. )
I played it for a while, if you have any questions about the gameplay I may be able to answer them. The reason I stopped playing was because there was SO much to do, it was addicting to me. Also, there is in-game currency (gems). (There is also membership, which I never had because none of the memberships were for life). Now, I have never spent a red cent on in-game currency--and I never will. Small quantities of gems are freely given daily, but, understandably without purchasing gems or a membership, it will be harder and take longer to reach your goals.
I was a pretty good player, earned many gems through battles, and joined a top clan which increased the amount of gems I earned (or would have if the glitch was fixed). I had a prosperous farm, and a nice little group of dragons. So, I gave my account to my sibling. If you ever decide to play, you can meet up with her, I can give your her friend code. (My sibling also plays Aetherlight. You have met her, but I won't say who she is for privacy reasons. )