A similar experiment to this has been done in a fluffy version of Harlow’s infamous Dependency in Monkeys experiment: Harlow’s Fluffies by Anonymunstah.
It has a different approach to yours in that a completely isolated foal is completely messed up as it has no socialisation whatsoever, despite the built in genetic programming for speech and key object identification (Mummahs, Daddehs, various toys, spaghetti, etc).
It might have been a mistake to have the soft plush speaking toys in this case, as they could potentially affect the foal’s behaviour and development.