Oh, poor Duncan!!! My Star is the queen butter eater. When I'm baking, she trys desperately to get the butter every time. A few times she's succeeded! Sounds like the butter was the culprit! Often, dogs, especially during holidays, become ill with upset stomachs from eating too much rich and fatty foods. Sometimes, it can result in an acute case of pancreatitis, inflammation of the pancreas from too much high fat food. I'd call the vet stat! Sounds like he needs to be treated with some fluids and medic! He may be becoming dehydrated. Please do call! And let us know how he is, ok??? Love and hugs to you and Duncan!!!