While they act like what you'd expect from a giant leech, the CR 6 Magical Beasts known as cave leeches at least have a novel appearance. They look like a giant leech crossed with something like an aboleth, with flailing tendrils whipping around their toothy maw. Still, they're kind of boring, and are basically leeches that happen to be able to tentacle-drag you before they suck your blood. They might have some refluffing potential, though...
If slaad didn't already make you unnerved by frog-like monsters in D&D, these CR 8 Aberrations are likely to. They are positively disturbing in nearly every facet of how they live. They can glide through the earth, meaning one can come up without even stirring the soil to blast you with a fear gaze, grapple you, and drag you down into the shadowy depths. Then, once they've taken you to their lair, they cocoon you in webbing excreted from their body, chew on your skull, and devour pieces of your brain while you are still alive. Then, to put the icing on the cake, they spit your brainless corpse back out of the earth, which then proceeds to become a wandering zombie. The cerebral stalker doesn't even control the zombies it creates, making it an even more eery mystery. It states in the text it's unknown how the cerebral stalkers reproduce; could they possibly use zombies as squishy flesh-nests for their tadpoles, which consume their shambling undead nanny before heading out into the world to find brains of their own? It sounds like a plot to me.
Another creature from mythology, yay! This time it's the church grim of English lore, also known as the kirk grim or kyrkogrim in the Nordic lands, one of several types of dog-shaped spirits found in the lore of western Europe. A pretty popular legend, considering it was strong enough to migrate with immigrants to America, taking the form of lore such as the Mississippi headless hounds and the black mastiffs of New England. They are the guardians of sacred land, created by a dog specially sacrificed and buried on the church grounds that they guard. They are defenders of the buried and companions of those that mourn the individuals that have passed on, but become vicious combatants against demons, grave robbers, and the undead. All in all, a pretty cool cat dog with a widespread folkloric background.
Thankfully, while only CR 5, the stats presented here for the church grim really does them justice as wardens of the cemetery. As incorporeal Magical Beasts, they aren't quite undead, but have several undead-like traits, including being bound to their cemetery and the ability to rejuvenate from destruction as long as their cemetery's church remains intact and its grounds hallowed. In addition, they have a mighty howl that causes evil creatures and ill-intented individuals to become shaken, and an incorporeal bite that both deals damage as a holy attack and inflicts 1d4 points of Wisdom damage. Last but not least is the increased casting DC of any animating spell performed in the grounds kept by the church grim, not to mention the immediate "bite me in the ass" target the caster paints on themself by using such a spell in detectable range.