There are number of animals which would be terrible to use to help Autistic children. The fact of the matter is, not every animal is friendly or displays obvious body language. An example of a dangerous animal would be large livestock which don’t interact with people generally and cattle specifically. One time when I was a teenager, I fed an older donkey an apple and it snapped at me. This is generally what animals which have not been trained to interact with people can act like. Cows have been known to kick people who approach them from the wrong angle when they go to milk them.
Large birds can be aggressive as well. There was a park in the small Missouri town which I lived in as a child which had a large cage with exotic birds inside such as Peacocks and Turkeys. I vividly remember putting a quarter into the machine, and getting some corn. Then, I went to feed them and a Peacock bit me! Needless to say, I learned to throw my corn through the cage and not reach into the cage. I’ve also had geese chase me and honk at me when I’ve been in the park feeding them bread.
Generally speaking, any animal which cannot be trained to interact with humans (specifically the children which this therapy would target), should not be used for therapeutic purposes. Commons sense would tell us that an animal which does not display obvious body language would be useless (although almost all do). So, this cuts out large birds as well as big, mean animals which people primarily keep to fatten and slaughter.