Foals are far more developed at birth than humans. I looked up the timeline for human development, and it appears that autism starts to become apparent around (or a little after) the time that humans reach parity with the physical development of a newborn foal.
It's pretty clear that normal foals are far ahead of normal human infants in musculoskeletal terms, but they're about even in terms of breathing, nursing, digestion, etc. (And human infants are far more effective at vocalization.) The sort of comparison you make reminds of the flawed concept of "dog years": there are no linear relations here. Anyway, the most troublesome symptom of a "dummy" foal is that it doesn't nurse properly. I haven't heard that about autistic children.
