Why is my puppy's shoulder wound now a pus-draining lump? What's next?

Hello and thank you for reaching out to Petco Pet Education Center, formerly Petcoach. It sounds like this wound that became a lump that is oozing fluid has become an abscess, or a pocket of infected fluid. Abscesses form when infection is trapped under the skin or within tissues such as fat or muscle. The bacteria will try to find a way out, so they can break through the skin and create a "draining tract" of pus. This should be addressed by your veterinarian, who will likely drain the abscess and open it up to allow continuous drainage. Antibiotics for at least 2 weeks will be needed, along with pain medications. Depending on the size of the wound/abscess, a bandage may be needed. I hope this is helpful! Thank you for seeking care for your Luna!