I've decided against going for Riding as a Supermax. Mainly when I saw one of the challenges '10 Horsey Best Friends' Yeah Right, as if I can even manage to get one! I'll probably do Nectar or something. I have plenty to choose from though so it's all good. They'll still have the Horse though.
Actually, this isn't that hard. I did a small test run of Riding, and here's how I did the Horse Friends one.
Step 1: Have both 'animal' slots open. More is better, but this is a dynasty. One of those slots will be closed for a stable competition horse, but the other slot will be full of rotating horses for this.
Step 2: Befriend you up a wild mustang until it moves in.
Step 3: Make best friends with mustang via spamming happy things. Maybe train while you're at it.
Step 4: Sell the wild horse.
Step 5: Repeat steps 2-4 until you complete 'Mustang Master'
Step 6: Buy or adopt a horse.
Step 7: Make Best friends with horse via spammage. Maybe train.
Step 8: Sell Horse
Step 9: You see where this is going, right? Keep doing this until you're done.
You'll also probably be further along in your career via this method, too, so it worked pretty well. It's pretty rough work, but it's managable, particularly with your buddies the moodlet manager and motive mobile.