This comment is low-effort, but in the case you are genuinely confused, the herd refers to the animals on a given ranch. As in, you have a ranch of 100 acres and 100 bison on it. The owner of the ranch owns a herd of 100.

The bison aren't roaming free on the land. It would be nice if they were, and there are efforts to restore wild bison herds, but these are commercial herds. Far better than cows and CAFOs.

One head of bison at a time.

The same way every beef farmer does.

Here's some good slice of life for you: https://www.youtube.com/@CrossTimbersBison

By owning (or renting) the land where it is kept ?

The same way you own anything. You buy it or make (breed) it.

First thing one need is herd mentality.

In order to own your herd, you need to herd your own

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