My trainer owns around 100 horses and is knows what she's doing. She might've been saying because she thought we wouldn't understand if she said bute... IDK.
But do any painkillers horses get have sideaffects like this?
ALL drugs have side-effects to them. They are foreign substances put into the body. The effect will vary from one patient (be it human or horse) to the next, which is why it is best to dose only under the direct advice of the medical professional familiar with the patient.