Exendin-4 is the natural form found in the gila monster. Because of the obvious difficulty in smoking a gila monster (they are impossible to keep lit, just ask Tom Isaacs
)as well as problems with enzyme degradation, the other forms are synthetic mimics but not exact duplicates.
The original research touted the anti-PD effects of stimulating GLP-1 receptors in the brain. When it was found that Ex-4 could do so (and lead to a patentable product) things zeroed in on it.