My dog's paw swollen for a month. Thorn or spider bite? See a vet?

Ouch, poor Zues! The swelling on his foot and "ankle" is quite pronounced and concerning. Since you mentioned that he likely stepped on a thorn, my biggest concern is that the thorn has embedded itself in his foot and migrated. Anytime foreign material (like a thorn or other object) gets lodged under the skin, it typically causes infection to set in, resulting in an abscess and localized tissue swelling/inflammation. The problem with abscesses around joints is that the infection can go into the joint capsule, which is a MUCH bigger problem. There are no effective home remedies to safely and effectively manage this type of situation. Zues really needs to be examined by a vet. It's possible that the vet may put a needle into the swelling to see what type of discharge is present under the microscope (white blood cells, bacteria, etc.). The vet will also be able to prescribe antibiotics and anti-inflammatory pain relievers, if needed. (Please do not give Zues any over the counter medications, as none are safe for dogs.) Until your vet visit, you can try to minimize his discomfort by soaking this foot in Epsom salts or using warm compresses. I hope that he feels better soon!