ME in Friesian breed:
https://thehorse.com/?s=horses+wioth+megaesophagus&et_pb_searchform_submit=et_search_proccess&et_pb_include_posts=yes&et_pb_include_pages=yes

Esophageal Problem Hereditary in Friesians? https://thehorse.com/152981/esophageal-problem-hereditary-in-friesians/

Megaesophagus, a chronic dilation of the esophagus in conjunction with a lack of normal tone or strength, appears to occur at an atypically high rate in Friesian horses. The report authors suggested that this might be considered a hereditary problem in the breed.
Posted by Stacey Oke, DVM, MSc | May 7, 2009 | Article

Megaesophagus, a chronic dilation of the esophagus in conjunction with a lack of normal tone or strength, appears to occur at an atypically high rate in Friesian horses, according to a report in the proceedings from the 10th International Congress of the World Equine Veterinary Association. The report authors suggested that this might be considered a hereditary problem in the breed.
Researchers identified an apparent increase in the diagnosis of megaesophagus between July 2002 and July 2007 in Friesian horses in The Netherlands. Siebren Boerma, DVM, Spec. Equine Internal Medicine, and Marianne Sloet van Oldruitenborgh-Oosterbaan, DVM, PhD, Dipl ECEIM, Specialist KNMvD Equine Internal Medicine, from the department of equine sciences at Utrecht University, relayed the research results.
Boerma and Oldruitenborgh-Oosterbaan recorded 45 cases of megaesophagus at the Garyp private practice and Utrecht University large animal clinic during the study period (2002-2007). Of these patients, 41 were Friesians. The researchers also noted a familial predisposition among affected horses, suggesting the condition might be hereditary.
Clinical signs of megaesophagus include loss of appetite, muscle wasting, salivation, and mild colic. Further, horses with megaesophagus are prone to esophageal obstruction (choke).
Management of affected horses can be costly, challenging, and can be associated with serious medical complications such as aspiration pneumonia (pneumonia caused by inhaling foreign objects into the lungs).

Abnormal Regurgitation https://thehorse.com/148330/abnormal-regurgitation/
What might cause my mare to regurgitate her grain?
Posted by Anthony Blikslager, DVM, PhD, Dipl. ACVS | Mar 3, 2014 | Article
Q.
I have a 15-year-old mare that regurgitates her grain. My vet and longtime horse friends have no idea the reason why she does this. It seems as if this could be dangerous to her throat and is very messy. Also, I notice she will also do this when in her stall and bored. I’ve had her vet checked and her teeth floated, and she has no problem with grass or hay. Vickie Bryan Wright, via Facebook

A.
This is certainly an unusual case, so I’ll go through it step-by-step.
First, the use of the term regurgitation is important. In small animals like dogs, an initial decision between regurgitation and vomiting would have to be made, but since horses are unable to vomit, it is the former. Regurgitation is essentially passive flow of ingested food in the wrong direction, and it localizes the problem to the esophagus (the same site as choke). In your horse’s case, a possible cause of regurgitation is that the esophageal wall is not working correctly, although a physical problem with the esophagus is also possible.
If you haven’t already had this done, I would ask your veterinarian to endoscope the esophagus all the way down into the stomach. This will require an endoscope that is three-meters long and is used to directly look at the lining of the esophagus. Specific abnormalities would be an out-pouching (diverticulum) that can collect feed, scar tissue that can slow down the passage of feed, or ulceration (esophagitis) that might be interfering with normal esophageal function. An esophageal diverticulum can be managed surgically, whereas the treatment for esophagitis would be medical, a lot like treating gastric ulcers.
Endoscopy can also be used to make sure the throat looks normal. You are correct to be concerned about regurgitation affecting the throat and possibly the airway because your horse may accidently aspirate food. However, you would have noticed coughing if this had occurred. Following endoscopy, I would recommend radiography. Because a normal esophagus does not show up on radiographs, I would recommend a double-contrast study, during which a nasal tube is passed into the entrance of the esophagus and used to administer a fluid that show up on X rays, followed by air.
This will likely be the most useful diagnostic because it can tell you if fluid passes down into the stomach at a normal rate, if the wall of the esophagus looks normal, and if the esophagus looks like it is normal-sized.
A couple of possibilities for your horse’s problem may show up, including megaesophagus (too large–and too flaccid), or a diverticulum that was not noticeable on endoscopy. The only way to deal with megaesophagus is through feeding management, which I will mention at the end.
If the esophagus appears completely normal, I think there is a behavioral possibility for your horse’s problem. I say this in particular because you mention that your horse seems to regurgitate when she is bored. Your horse may be using a reflex similar to cribbing in which swallowed food can be actively regurgitated so long as it’s fluid enough. This may give pleasure in the same way that swallowing air and regurgitating does with cribbing, or at least relieve boredom. Horses can learn to crib without the need to bite a fixed object like the stall door. The reason this would not happen with grass and hay is that the roughage prevents the ‘slurry effect’ that occurs with swallowed grain.
As far as a way to manage your horse’s problem, you have noted that the problem does not occur after eating grass or hay, so you could try switching your mare to an all-roughage diet. Instead of grain, you might consider a high-quality hay in combination with a feed such as soaked beet pulp. This might take some time to figure out, depending on your horse’s activity level and condition. Overall, if you are able to rule out an esophageal problem that can be treated medically or surgically, I would then strongly encourage you to work with your veterinarian to see if there is a dietary fix for the problem.