Can we have the most feared monster of medieval times come along to wurm.
It could also be another monster that allows the collection of Cochineal.
That and also could create a shield out of the shell.
It could also award a humourous title when you slay one of them.
If you want to make them more dangerous, you could give them slime power which makes the attacker lose some focus or hit rate and also make them more rare then other monsters.