Madison: “I know he’s been cheatin’ on me, I can just tell. I’m gonna have words with him.”
Bonny: “Now hold your horses! You don’t know anything for sure ‘till you’ve spoken with him.”
Madison: “I just know he has. Dirty, lyin’ cheater.”
Bonny: “Your call. Still think you should hear him out though.”
It is often used when somebody is being hasty, or rushing into something.
Texas has many ranches; the phrase is likely to have originated in the horse breeding community.