This is quite unfortunate. As one might receive grace by partaking of the Body of Christ, one may only receive sin and wickedness by partaking of the Body of Man. Hence come all the monsters of the nights and wild places - beasts that have feasted upon the flesh of men, violating the dominion granted in the Garden are transformed. Sin builds up like poison within their blood and bones until all trace of their rightful station has been wiped away and all that remains is their acting-out of man's stolen depravity.
I have listed here only those beasts I have seen with my own eyes, have heard of from trustworthy eyewitnesses, or have read about from learned and worthy sources. It remains incomplete. Satan's creativity in his perniciousness knows no bounds.
Boars shall be made into the bristlebacked hordes of the orcs. Of all monstrous beings they are most common; a thundering, squealing wave of pillage and rapine. Of war and implements of war they have inherited a great love, though they can make no such weapons on their own, and lack knowledge of any other craft or tool.
Dogs shall be transformed into the terrible barghest. The black moor-hound may reach the size of a pony, with claws like that of a bear and eyes like burning embers. Its shaggy black fur, matted with filth, is as resistant to a blade as good armor.
Wolves, however, shall be transformed into the loup garou, a beast which takes on a more manlike shape than its common cousin. They do not walk according to their form - bones and skin rebel, and go different ways. I met once a French Benedictine who claimed to have seen one in the midst of its moon-frenzy, and to have witnessed its demise at the hands of a novice who cracked the fiend's skull with a silver candlestick.
Tigers of the east, upon devouring a hundred men, shall be transformed into manticores, who bear men's faces and speak with men's voices. In Jerusalem I met a merchant of Babylon, who has spoken previously with Indian merchants who sell the steely, poisoned needles of its tail as weapons of assassination, and the pearls of its stomach as ingredients to elixirs of life.
Lions shall transform into chimeras. Like a king beset by unworthy advisors, so the noble beast is beset by the arguments of the goat (being the sins of the common throngs) and the dragon (being the sins of the mighty.) and is thus perpetually at war with itself.
The hyena of Ethiopia shall become as a gnoll. I know little of this beast, save that the mere hint of its laughter makes those inhabitants of the region tense, and that I saw one's hide for sale as a rug in Alexandria.
Bears will lose all of their fat, becoming little more than sacks of skin. They will carry around animal hides and stuff stolen children into them, to eat later. The skull and jaw split apart, becoming as the mandibles of an insect. The common folk call the creature a bug-bear. Where practicality reigns, creativity stands aside.
Rats shall grow to the size of a child and stand upon their hind paws, burrowing into the walls and below the foundations of manors and keeps.
The jackal shall become one of two forms: the cynocephalus of India, and the 'goat-sucker' as described by certain scholars of Aragon. The latter is more obscure, but is said to retain a dog-like shape, gaining a leprous appearance and a taste for the blood of livestock.
A lizard might in due time become a basilisk. Several bestiaries I have consulted claim that the tarrasque was a basilisk that had grown since the time of the Patriarchs, but all are conveniently vague on the matter of where its bones are interred.
I heard of only one instance of a man devoured by chickens, but I have also only heard of one instance of a town overrun by cockatrices. The link is sound.
Serpents become dragons, a most natural extension. Certainly it is the serpent of the Garden who, after having eaten its fill of the lost and damned, becomes the red monstrosity on which the Whore of Babylon sits astride.
Insects grow to prodigious size and ferocity, but rarely change their forms - the scarlet deathworm born of certain breeds of maggot is the primary exception.
Sharks and fish shall become as the Deep Ones of the cold northern waters, and all manner of sirens and mermaids besides. They prey upon man's lusts and drunkenness, drawing the foolish into their saltwater graves.
Of the Nile crocodile there comes the lizardman, so sits indolently upon the bank with birds in his teeth. He cares not that you might see him, nor even that you approach, for he considers you well aware of your inferiority already; he does not need to remind you.
Men who devour men show the greatest variety in evil forms.
- Those that eat flesh out of gluttony become ogres. It is an affliction of the first estate, as a man without wealth will find it hard indeed to glut himself as a baron would.
- Those that partake out of starvation become wendigos. Most common they are in the cold wastes of Ultima Thule and the deep deserts of Arabia. Pitiable, were they not most deadly.
- Those that eat only the dead become ghouls. Dwellers of crypts and ossuaries, bone-gnawers.
- Those that are driven by the throws of leprosy of syphilus or the plague become zombies. Half-dead and dead of mind, they shamble about in search of warmth before they can go no further.
- Those that drink of man's blood and blaspheme against God, recanting of His mercies, become vampires. They are struck dead upon the spot, but ride every night to hunt the living.