Wesen in the F category.

A Folterseele (FOHL-tuh-zay-luh)
is a frog-like Wesen. Folterseele are known for being always beautiful,
yet always deadly. They do not woge like any other Wesen; they only woge
as a defense mechanism when someone is sexually attracted to them. They secrete
batrachotoxins from their skin, which is most closely related to that of
the Golden Poison Frog (Phyllobates terribilis). Any contact with this
toxin is extremely lethal, and as little as one ounce is enough to kill 10,000
mice, 10 to 20 humans, and 2 African bull elephants. The toxin induces acute
systolic heart failure. Due to this defense mechanism, they are a rare Wesen.

One way to neutralize the secretion of a Folterseele's poison is to depolarize the cell membranes by using a neurotoxin (substances that are poisonous or destructive to nerve tissue) such as tetrodoxin or saxitoxin. These neurotoxins totally block the creation of the poison. A side effect of this treatment, however, is that
the Folterseele's greenish skin color becomes partially visible to all,
particularly in the area of the face, and is left with a dry, almost
scaly-looking appearance. However, this is the only woged feature of a Folterseele that becomes visible to all after the treatment.

Many men have difficulty controlling themselves around a female Folterseele, and they have a higher risk of being the target of rape attempts. Because of this and what happens to those who contact them when they woge, Folterseele mostly keep to themselves and avoid romantic relationships. Some go as so far as to disfigure their faces to make themselves less attractive, but they also struggle with never being able to experience love.

Features:

When they woge, they gain a frog-like appearance in which their skin turns a yellowish to brownish-green color, their eyes slightly bulge and move slightly further apart, their nose bridge flattens, and their mouth and lips widen. The lower jawbone is reduced to just the lower lip, and the throat becomes an inflatable vocal sac.

Fuchsbau:

A Fuchsbau (FOOKS-bow) is a fox-like Wesen. Unlike Blutbaden and other canine Wesen, Fuchsbaus do not have attack superiority over humans in the attack department, although their jaw muscles are powerful enough to at least tear off a chunk from their victims, just not enough to cause significant damage. They are known to have an excellent sense of smell that is in the same league as that of a blutbad, and can even tell what gender the individual they pick up the scent of it. Their sense of smell becomes stronger when they woge.

Generally deemed as "liars" by the Wesen community, Fuchsbaus may deal with shady transactions, including the buying and selling of human organs. Despite this, Fuchsbaus are loyal and friendly to those whom they consider friends. 

They generally prefer to avoid confrontation; however, they will fight when forced to do so and will not go down until they at least succeed at biting off something from their adversaries. They prefer to ambush and surprise those who come after them.

Fuchsbaus do fear Grimms like most Wesen. Their first response to meeting one is usually to try to run away, but they are willing to at least work with them after being assured the Grimm means them no harm.

Features:

Compared to many canine Wesen, Fuchsbaus have a mild woge. They grow long, orange fur, as well as either white or black patches all over their body. Their facial features remain relatively the same, save for becoming slightly more pointed. They also gain sharp canine teeth, fox-like ears, fox-like eyes, and a black, fox-like nose. Their hair stays roughly the same.

Fuchsteufelwild:

A Fuchsteufelwild (FOOKS-toy-fuhl-vilt) is a goblin-like Wesen. Fuchsteufelwild are highly intelligent and clever, which allows them to concoct their highly elaborate plans.

 Fuchsteufelwild are highly sadistic and love to play intricate games with their victims. While the nature of their games varies, they all have one thing in common: the victim must guess the Fuchsteufelwild's name, which is always an anagram of the letters that make up the name Rumplestiltskin (i.e: Kilters Pintslum, Stumps Riltecni, Mike Trillspunts). Due to their enormous egos, Fuchstefelwild will become violent if they feel like they are about to lose, and they will frequently try to kill their victims. If they know there is no way they can win, Fuchsteufelwild will often commit suicide. Despite their formidable offensive capabilities, Fuchsteufelwild prefer to take on their prey one at a time via ambush, likely because of their small stature. If faced with multiple well-armed foes, Fuchsteufelwild will run. Fuchsteufelwild are particularly terrified of

The nails of a Fuchsteufelwild drip a highly corrosive acid, which is generated from the muscles in their hands. When the acid is used in conjunction with their nails, Fuchsteufelwild can cut through flesh like a hot knife through butter. The acid is also strong enough to cut through much tougher substances, such as wood, glass, and even solid metal. Despite being small in stature, Fuchsteufelwild can leap incredible heights and distances, even while in human form. They are also very fast and agile and can move with great stealth.

Features:

When woged, Fuchsteufelwild gain green skin and sharply pointed ears and noses, as well as fangs. Most notably, their four fingers fuse into two large digits from which extend razor sharp fingernails, which are semi-retractable, and a thicker thumb. Their voices also become higher and more shrill.