What Causes Canine Foaming At The Mouth, And Complete Lethargy?

Rabies is very uncommon in domestic dogs and cats.  If your dog has not been vaccinated and experiencing other neurological signs then rabies may be a rule out.
When dogs are uncomfortable or nauseous they will often drool excessively.  This drool can be very foamy.  A taste reaction will also cause very excessive often foamy drool--this occurs when they lick something that tastes very bad.  Taste reactions are not usually accompanied by severe lethargy.
Your dog needs to be examined by a veterinarian to determine whether the lethargy and drooling are related to pain or nausea.  This will guide treatment and get your dog feeling better.

Typically foaming at the mouth is related to rabies, but if the pup has had it's shots, then the likelihood of that is low to none. There is something very wrong with the dog, and you need to find an emergency vet to get it too, sorry to say it, but you didn't specify what sex the dog is. Hope this helps you out. Good luck!

I'm not an expert, but raibies maybe?