Animals that aren't dangerous that Planco AI could walk through their exhibit. E.g. "Lemur walk way" Countless zoo's in the UK have these lemur exhibits where you can walk through on a path.
I think only the peacock was allowed to free roam. But guest couldnt see them..I agree!
In general, just some animals that don't have a "dangerous behavior" so to speak; so even if they do get out, guests won't freak out too much.
I once tried to make a zoo in ZT2, where I wanted some free-range/walking peafowl.. but nope, guests started screaming and running away, and zookeepers tranquilized them.