Hi Kimberly,

Have you read the file on Metformin here: https://ecir.groups.io/g/main/files/4%20Insulin%20Resistance/Metformin?  If your horses will not eat the Metformin you will have to syringe it and rinse their mouths out afterwards.  That is a much more reliable deliver method than adding to feed as it seems most horses do not eat it willingly. 

As far as Mouse being on Bute - lameness due to elevated insulin will not be impacted by any NSAIDs.  What it will be impacted by is getting the insulin level down through diet and possibly medication and getting the trim corrected.  Since you're taking all of those steps you can wean him off the Bute.