Jobbernowl is a real word - I found it in an old OED we have kicking around the place. And a jobbernowl is definitely a creature to beware of. However amusing it might appear.
Thanks to Kim at Larvatus Prodeo for the link to this specimen of the species. Which led me on to this fine example of what happens when you tangle with a jobbernowl who's got things completely gurmbled.