So he has a lump behind his neck and I've been gone for about two days, when I left there was no bump, his diet hasn't changed nor his behavior. Pet's info: Dog | Mixed Breed | Male | neutered | 1 year old

Any bump that appears quite suddenly like that should be evaluated by a veterinarian. It could be an abscess, or a cyst, or a tumor. The vet will likely start with a fine needle aspirate, which is where a needle is inserted into the lump and a sample of cells is taken to look at under the microscope. This will help to diagnose exactly what is going on with Zeus.

It could be an abscess, cyst, or an enlarged lymph node. He should be examined by a vet to determine the exact cause. Your vet can do a fine needle aspirate to diagnose and then determine the best way to treat. He may just need an antibiotic to treat an infection.

That looks like some sort of skin mass on Zeus. It could also be an infected hair follicle. In general, skin masses are benign but there is always a chance it could be something cancerous. You can take him into the vet to get it checked. This is the safest option and the doctor is likely going to recommend to aspirate the mass to look at the cells. Thats the most accurate way to know if it is dangerous or not. Otherwise you need to monitor the mass very closely, if it changes in size then I would recommend to get it checked. An infection could present like this and the lesion may burst. Clean the area gently and make sure Zeus cannot scratch it. It will likely clear up on its own but take him into the vet if it starts looking swollen or is bothering him. I hope this helps!