There is a problem: 2 different themes have one common tag: "cowgirl"
1) Cowgirl - from "cowboy" (girl, which dressed as cowboy).
2) Cow-Girl or Girl-Cow - a girl and a cow in the same time (maybe she is dressed as a cow, or she is a furry)

Actually, we have technically separated those into their own tags when a similar thread popped up long ago.

The problem now is that many of the images and tags probably haven't been indexed into their respective categories yet. That responsibility basically falls upon all Gelbooru users at large, so you are welcome to do some work on it if it is bugging you.

gelbooru.com/index.php?page=wiki&s=view&id=6101

Proper tagging for each is explained here. (The fact that the example images to the right of the wiki page are incorrectly matched is proof that this is a work in progress)

So, can I replace the "cowgirl" tag by "cow_girl" for furry/cow-cosplay images?
If I right understand, the cowgirl tag is incorrect for furry/cow images, but the cowgirl is still correct for the "cowboy"-girls?

Yes.

Edit: Never-mind. I took care of it for you.

That tag that she dress as cow or she is furry is called holstaurus

No, the tag is cow_girl.

holstaurus is a specific type of cow_girl. Specific enough that it's considered to be a specific character rather than a general cosplay/furry.