In Dungeons & Dragons, hydras are classified as monstrosities and not dragons. Lenmos didn’t like this classification, so they decided to do something about it and they created the unofficial supplement Dragon Hydras. This supplement offers 12 new monster statblocks for the new dragon hydras. They’re pretty cool to look at and will definitely provide a challenge for your adventuring party.
Why aren’t hydras dragons? I don’t have an answer but, I do have a solution.
You can purchase Dragon Hydras from DMs Guild for any price you see fit.