Hello and thank you for the question.
Let me begin by saying that there are no venomous snakes in Iceland and only one very rarely found in Scandinavia so the recurring theme of venomous snakes in historical Icelandic texts seems incongruous at first. However, what is commonly translated to mean “snake” in English, is often less clear in Old Norse. The actual word used (in Old Norse) is “ormr” (often referred to as “wyrm” in English).
The quote from Gylfaginning is as follows:
“Þá tók Skaði eitrorm ok festi upp yfir hann, svá at eitrit skyldi drjúpa ór orminum í andlit honum, en Sigyn, kona hans, stendr hjá honum ok heldr mundlaug undir eitrdropa.”Ormr is commonly a serpent rather than a “snake”. Although the two are very similar, a serpent is more commonly seen as a mythical creature. Further examples would be Jörmungandr (otherwise known as “midgardsormr” and who spits venom) and also the serpents that are said to drip venom on the unworthy in Náströnd in the description from Völuspá:
Old Norse – “Fello eitrdropar inn um lióra. Sá er undinn salr orma hryggiom.”
English – “There fell drops of venom in through the roof vent. That hall is woven of serpents’ spines.”Even in modern Scandinavian folklore, the Linnorm is known for it’s corrosive and poisonous venom that can be spewed as well as bitten.
So from all of that, we can reasonably assume that we are discussing a mythical serpentine creature with a corrosive venom rather than a typical snake as we understand it today.
I hope that helps.