pasture
the field
In a field where it is full of grass and trees with fresh air/water.
It is commonly called a pasture, paddock, or field.
Agricultural horses can be called a team, Race horses can also be called a field Regular horses are more commonly called a herd
A group of wild horses can be called a field, a herd, a set and also a remuda.
A small field for horses should provide enough space for them to move around comfortably, graze, and engage in natural behaviors. It should also have access to clean water, shelter from the elements, and be securely fenced to ensure their safety. Regular monitoring and maintenance of the field is important to ensure the horses' well-being.
The common word is 'orchard' or 'grove'.
Generally if there is food available horses will eat it. When horses eat, their body produces heat...so if it's cold at night make sure that your horse has a nice blanket and a big full haynet. Horses are grazers, so if your horse is out in the field during the day, he will graze.
Depending on the size and location an enclosed field can be called a pasture, paddock, field, or pen.
the field
in a field
Horses are just called "horses" unless they are in a group together. When horses get in a group together, they are then called a "herd".