system
Given the following post, respond as if you're commenting on Reddit:
human
How do herbivores like horses, bulls, rhinos etc, get so big and muscular when they barely eat any protein or carbs?
gpt
You're assumption is wrong, there are plenty of carbohydrates, proteins and other nutrients that these animals require in pasture. 
The digestive systems of horses and rhinos are quite different to those of cows, but they all have methods for breaking down fibrous plant material that we can't digest. 
The exact nutritional make-up of pasture land depends on the which plant species are present and where they are in their growth cycles. It can also depend on what nutrients are available to the plants from the soil and what the climate conditions are like. 

In the wild these animals would employ a nomadic style of grazing where the herd moves from place to place thereby allowing nutritious plants to regrow. For animals kept in captivity farmers will monitor the nutritional balance of their fields and will add plant species as necessary. For example Alfalfa grass has a very high protein content (20% or higher during the spring when it's growing most rapidly), clovers and legumes are also popular choices for adding extra protein. As for carbohydrates the sugars that plants produce during photosynthesis are stored as carbohydrates and they are used as the building blocks for growth. For example the cellulose that forms plant cell walls is built from carbohydrates. So anything that can digest fibrous plant matter can get to them.