Great question! There is a growing body of evidence that some animals do, indeed, ingest plants for medicinal purposes. The study of such behavior is called zoopharmacognosy. Most documented examples pertain to primates, but the Navajos claim to have learned the use of a wild parsnip root as an antiseptic from bears and the phenomenon may be fairly widespread both geographically and taxonomically. The theory is that if animals have learned over time that certain foods make them sick, they are likely to have learned that certain other foods make them well. This is a very new field of study with a tremendous amount still pure speculation.