I've introduced mine to as many different animals as possible, we did sheep via a farm ram whose always used for the farms pups he give them a little sniff usually headbutts them and the dogs are forever wary after that, cows I did both calves and adults, lama (don't ask monty is friendly) various birds, horses, other dogs, cats basically as many different things as I could. Really pays off later on. The park here used to have calves and goats kept back as part of a petting zoo and they were great for socialising because they were used to people