Once again, people being surprised by a hydra head growing back, yet knowing what a hydra is in the first place
To be fair, if we go back to the original lore, hydras just were many-headed. The regrowth was added more than three hundred years later to give Iolaus something to do to assist Heracles (and make the tale more fantastic to tell, for a tragedian), and it was specific to just the Lernaean Hydra, as well. It's mostly modern depictions of hydrae that make it a racial trait and only if the medium allows it (many older games or those built around sprites don't usually let you sever heads, for example, so it's not depicted in those), often combined with some form of high natural regeneration; the extremely poisonous blood is rarely referenced, unless it's explicitly a poison hydra. Also of note is that Heracles's labours is really just the Greeks adopting and localizing Sumerian/Babylonian/Assyrian mythology, as Ninurta (war/hunting god) also had 11 conquests over Tiamat's (who is the Sumerian original from which Greek Echidna is formed, the Mother of Monsters) offspring in the praise poem "The Return of Ninurta to Nippur".