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Well, it could be alittle of everything. Many times it is a series of events as they get older and they seem to go from good to bad in 24-48 hours. As one thing "fails" like the heart...it all goes.
I wouldn't rule out cancer, but at 16 yrs of age, I am sure the heart/liver/kidneys are all working pretty hard.
The drooling and foam can come with nausea, discomfort and where the body is just working on breathing. The labored breathing is usually just to keep the body oxygenated. But can also be labored from fluid in the lungs from heart failure...as well as pain.
The legs can be from decreased blood flow, spinal injury/lesion, other.
I know that this weekend will be hard for you. Based on how he does you may want to decide on Monday whether the test are important or not.
Keep him quiet and comfortable and away from the other dog so that he is relaxed.
I will keep you all in my prayers.