I'm not saying I don't occasionally swear at my lovely mare. I do. Especially if she's attempting to (or worse, succeeding at) bolt under saddle. But, it's more like "GDit, stop that!" or some such phrase. Not, "I'm going to send you to the knackers", because whether she can understand my words or not (and we'll not put that up for discussion), it sets up a negative scenario in my mind. The reality is that I'll never sell her...I just don't want to put the thought in there.
On a similar note, I used to board her at a barn for the winter and once while I was in the crossties getting ready to ride, one of the young riders heard me asking her "foot please...thank you" and commented that I was so polite to my horse. For me, it just sets up the relationship to be positive. I wouldn't actually take no for an answer, but I feel better acting/speaking that way than ordering her around. :::shrug:::
I knew a farrier that would actually DO that... if a horse bit him while he was shoeing a front, the farrier would bite back!! (usually on the horse's shoulder). Surprise the hell outta the horse, and they usually did not bite the farrier again!
If my gelding is misbehaving, I usually threaten him with "Do we have to have a Come to Jesus meeting?"!