Is there a word for large group of horses? I know there's "flock" for birds, but I am wondering if there's a similar word for horses and land mammals.
The word you want is herd:
1 a : a typically large group of animals of one kind kept together under human control
// a herd of cattle
1 b : a congregation of gregarious wild animals
// herds of antelopes
I see a flock of birds and a herd of horses.
In addition to Jason Bassford's excellent answer, you could also say:
1. Troop: A group of people or animals of a particular kind.
Example: A troop of horses.
2. Stud: A collection of horses or other domesticated animals belonging to one person.
Example: He kept a large stud of racehorses.
A stud of horses implies that the horses are kept specifically for breeding. You can't use stud in general sense.