I agree with what many here have said, esp not mixing bute and previcox. Both are anti inflammatories, with previcox being somewhat safer, far as GI effects, being a Cox 2 inhibitor
Previcox is also being used off label to treat horses, being for dogs, while the same chemical drug formulated for horses is Equioxx Previcox vs. Equioxx: What's the Big Deal? - IrongateIrongate
Anti inflammatories also are just pain relievers, and do nothing far as joint health, and there are better products available, that address the actual joint issue, from joint injections to something like Legend
Here is some things you might consider, if your goal is just not pain management, but actual joint treatment, which perhaps can delay, even improve joint chnages like osteo arthritis Choose Your Joint Care Weapon Wisely | The Chronicle of the Horse
If you are going to double up on treatment, amke one a pain reliever, and the other a joint support medication
However, arthritic pain is no excuse for general disrespect, like your mare is showing.
Sure, changing barns, having new equine companions adds a certain amount of stress, however, still do not excuse general lack of respect
Horses also become untrained, well as trained. They test new people, and first just take slight liberties, then if not corrected, escalate
NOw, not sure how I want to put this, without sounding harsh, so I will just dive right in.
I think it is great, doing everything possible for an old faithful equine companion, making their last years have quality-horses that served you well for years
At the same time, you bought this horse, hoping, I believe, to do more then pay board, treat lameness issues, ect.
I would find a retirement home for this horse, and then pay board, invest time , on a horse you can truly enjoy