Mental illness in what way? Animals have physical brain deformities just like some humans, which resorts in behavioral and nervous issues. Last time I checked you can't psychologically diagnose an animal because they do not have the cognition of humans (try giving a cat an ink blot test...) upon which our collective knowledge is based. Professional neuro researchers regularly use sheep, mice, and monkeys (not very common), dissect their brains and check for abnormalities...that's actually part of getting a degree in the field.

__________________
TC
Current stable:
- 2004 Mazda RALLYWANKEL
- 2007 Saturn sky redline
- 2004 Explorer...under surgery.

Past: 135i, GTI, 300E, 300SD, 300SD, Stealth