A group of horses can either be called a band or a herd.
A band.
colony
A system
A flotilla.
A school of fish.
A group of lions is called a pride.
If you mean "group", it is a covey.
A group of whales are commonly called a pod, or a school.
A group of chickens is called a flock, kinda like other birds. :)
genuinely a cluster, but some have developed their own names for it.
a herd
they are called mustangs. they are the American wild horses. Australian wild horses are called brumbys.