A member of the public found the female Jack Russell terrier with part of her scalp and an ear missing on May 23, in a park off Broad Lane, in Walsall.
The dog has since died from her injuries. The RSPCA suspect a larger dog could have caused the injury.
RSPCA inspector Fiona Howell, who is investigating, said: "We don’t know exactly what has happened to her but vets suspect the injuries were caused by another dog significantly larger than herself.
“She was found alive in a park by a passerby who contacted us for help. Unfortunately she has since passed away from her injuries.
"She doesn’t have a microchip so we’ve been unable to track down an owner so we are appealing for anyone who has any information to contact us in complete confidence on 0300 123 8018.”