Yesterday, within a couple hours, our chihuahua developed a walnut sized lump on his neck. It is kind of hard but also feels soft in a couple places. We do have a spider problem, and had fox tails in the yard when we moved here last month. He does not cry when we touch it, and does not act like it bothers him.

It could be an abscess. Abscesses show up over night.. He may have gotten poked or maybe a foxtail migrated into his skin. Either way, he should be seen by a vet. Abscesses do not go away on their own, especially if there is something in there bothering it.