A rabbit dewlap refers to the pendulous flap of skin that hangs beneath the chin, around the neck, or throat area found mainly in some female rabbit breed with a few males having it too. However, the one on bucks is often smaller, i.e., not as pronounced as the one that does have.
According to the ARBA Standards of Perfection, some breeds can have a dewlap whereas it is not is permitted in others. The dewlap can be simple, prominent, or overly large. Some bunnies may have a double one that forms a ‘W’ like fold instead of a ‘U’ like shape.
A dewlap is common in larger breeds. However, some smaller breeds have it too. Does of breeds such as the Holland lop, Flemish giant, French lop, American, Beveren, Giant Chinchilla have it while some, such as the English Lop and Florida white have it in both male and female.
Also, some breeds with loose skin including Giant chinchillas, the lops, and Flemish giants tend to have larger dewlaps. However, it should not be confused with an obese rabbit as it is only on the throat area and not the whole chest area as observed in overweight bunnies.
Role of dewlap
This flap is made of fatty tissue (stored fats), and it is associated with some female sex hormones. Therefore, it develops as the bunny reaches sexual maturity. Males that have it are associated with the presence of small amounts of female sex hormones.
For instance, research shows that the dewlap does not develop in does that are ovariectomized before they reach their sexual maturity. On the other hand, the “administration of a butyl alcohol extract from the urine of pregnant does induce a pronounced dewlap even in male animals.” 
It is worthwhile noting that this skin flap is not a sign that your rabbit is obese or overweight. However, it is common in overweight spayed does and in some bucks.
One of its roles is to provide fur for nesting, i.e., “while pregnant, the female rabbit will pluck fur from the dewlap to line a nest for her babies . During this time, hair pulling is normal. However, you need to ensure there is no much underneath skin damage.
Wet dewlap and other challenges
One of its problems is abscesses  associated with repeated trauma, owing to its location. The use of feeding and water crocks or metallic feeders that have rough edges can cause trauma as they will continually rub against the dewlap surfaces and this can result in injuries.
The problem will be worsened by a wet dewlap, i.e., it may get damp or wet as it is continually rubbed on wet surfaces including it getting into the watering bowls. Wetness can also be as a result of drooling caused by malocclusion as well as a damp surface on its cage or lying on wet bedding substrates.
A wet dewlap can cause moist dermatitis (a kind of moist skin infections) characterized by a wet mattered fur, gangrenous smell, hair loss, green skin, and presence of abscesses. This place can also be a target of flystrike.
Injuries and the dampness can encourage bacterial growth as well as infections, especially by the P. aeruginosa.
Treatment and remedies
To avoid trauma and wetness, raise your rabbit feeders to force it to stand by its front feet to be able to access food over the feeder’s edge. Also, consider using rabbit water bottles which features an automatic watering nipple.
Additionally, placing a well-sized plastic ball or rubber can help keep the dewlap dry if you use crocks. Moreover, you can opt to cover it, leaving only a small hole in the middle, about two inches in size, where it can drink water from, and thereby prevent the wetting problem.
Clipping the hair, applying a broad-spectrum antibiotics ointment until the area heals are some of the treatment options you could adopt.
Finally, ensure your cage is dry and clean. Remove any soiled bedding, straws or hay.
A dewlap is harmless and should not worry you. However, you need to ensure that some of the associated risks including moist dermatitis and trauma are prevented and managed.