Morkeas

DM: "Finn, you've heard of these creatures before and they're very dangerous."
Party: "More gay ass, got it, very dangerous"

Morkeas are huge snake-like creatures capable of breathing out a jet of fire, almost like a dragon. The Morkeas secrete some kind of mucus which allows them to create a kind of armour (ok it's a mech suit) which not only protects them from damage, but also from spell effects which would target them specifically, such as Phantasmal Force.
A young Morkeas was found attacking a group of wagons on the road, and was responsible for the death of May's family. A much older specimen attacked the group further into their journey and very nearly killed Guy.