Rabbit cysts refer to pocket filled with fluids (gaseous, semi-solid and liquid materials). They are closely related to abscesses in rabbits. However, in abscesses, there is always an infection whereas cysts are not usually infected, but it is possible for infections to occur turning them into abscesses.
It is also a good idea to distinguish them from tumors. Rabbit tumors refer to abnormal cell growth leading to masses that may be malignant (cancerous) or non-cancerous (benign tumors).
Finally, cysts can affect any part of a bunny’s body, both internal and subcutaneous. They can appear almost anywhere including beneath the skin, on the uterus, testis, head, and so on. Let us look at some of the common ones that this animal may have.
They develop beneath the skin and are sometimes referred to as keratin, epithelial or sebaceous cysts and they can appear any part of this pet’s skin.
They are often small and feel like a hard lump and are not cancerous. They are formed due to a buildup of keratin, i.e., they “ lined by stratified squamous keratinizing epithelium and filled with concentric layers of keratin” 
Treatment is not required unless they keep on increasing in size or have other symptoms especially if they get infected. Sometimes, they can go away on their own.
Coenurus serialis cysts and cysticercosis
The Taenia serialis is one of the tapeworms in rabbits whose larvae stage, Coenurus serialis, forms cysts in the subcutaneous tissues characterized by a soft, fluctuant swelling beneath the skin.
On the other hand, cysticercosis refers to those formed in the abdominal cavity by the larval stage (Cysticercus pisiformis) of the tapeworm Taenia pisiformis.
Mastitis cysts (mammary gland)
They are found in “mammary glands; contain clear to a cloudy fluid.”  These sterile cysts filled with fluid are formed inside the mammary papillary duct and could be one (as in most cases) or “coalesce, with fibrous tissue forming around the cysts, and may end up involving several glands” 
Treatment is by ovariohysterectomy and use of hormones. These cysts are often associated with uterine adenocarcinoma and also noted in bunnies with endometrial hyperplasia, pyometra, and endometritis
These are fluid-filled sacs that occur along the spine whose treatment is surgical intervention. They can result from spondylosis of facet joints 4] and they may cause various symptoms including pain.
Follicular, ovarian cystic rete ovarii
As the name suggests, they are in the ovary although they may be on the rete ovarii area. They are commonly associated with endometriosis, hormonal issues or pregnancy.
Diagnosis may involve palpation and imaging depending on whether they are internal or subcutaneous.
Depending on the cause, removal is often via surgery under general anesthesia especially if they are growing or they cause pressure on nearby organs.
In most cases, they may not cause a major issue. However, if they get infected or rupture, they may be painful and messy.
Finally, for those related to the reproductive system (mammary and ovarian ones), neutering your rabbit may help reduce their occurrence.
We have not exhausted all the types of rabbit cysts but rather given some of those that do occur including the cerebral, renal, among others.