Not only humans, even animals need dietary fiber. I recently came across this article which mentions how horses also need dietary fiber. You can read this article for more information. If you are interested in learning more about fiber needs in dogs and cats you can read the articles here and here.
With advances in nutrition and preventive health care, horses can expect to live longer, healthier lives than they have in the past. But, as horses age, it becomes more challenging for them to chew digest hay and grain, which in turn makes it difficult for them to maintain a healthy weight. Fortunately, owners have options when it comes to finding a palatable feed that older horses can chew and digest easily, such as senior horse feed.
Dental problems are a common problem among aging horses. Poor dentition creates a snowball effect of health issues that leads to malnutrition, which inevitably leads to weight loss. Thus, having your older horse on an appropriate proper diet is essential in prolonging their health.
Typically, senior feeds are complete feeds that allow a horse to get all of their dietary fiber via feed rather than through forages. Southern States equine nutritionist Marty Adams, PhD, said, “A senior horse feed should be highly palatable, easy to digest, based on digestible fiber instead of grain, low in sugars and starches, as well as high in protein and fat.”
Senior feeds are typically higher in fat and fiber compared to other feeds. The fat adds calories to promote weight gain, and the fiber replaces what a horse would typically get from consuming forages. Higher fat formulas also lead to a boost in energy that older horses need.
“Although most senior horse feeds are high in fiber, and can be fed as complete feeds with no added fiber, these feeds can still lack what is called the ‘chew’ factor and the ‘scratch’ factor that will provide increased chewing and salivation,” explained Adams. “Even if your horse has few or no teeth, hay cubes or chopped forage can be soaked until soft and mixed into feed or fed separately to your older horse.”
Source — http://www.thehorse.com/articles/34442/understanding-senior-horse-feeds