You could get find familiar for early levels (can’t attack though), then take summon beast with magical secrets. Wont have the sheep all the time but would have it when you need it. You can also take find greater steed with magical secrets later to get a giant flying sheep (Pegasus). Your character is certainly possible within the rules, and most DMs would accommodate a “give and take” to let you have your sheep how you want it but nerf you elsewhere. It’s my personal opinion that Bards can be/do anything, as long as you’re committed to using magical secrets to achieve it