As promised in this post, here are the experience and quirk tables for dog and horse animal companions, suitable for use in any OSR-ish RPG. If you're looking for mules, check out the previous post.
Dogs and Guard Dogs
Dogs include two categories of animal companions. A basic dog is your typical house pet or mutt, lovable and friendly. Guard dogs are larger animals that are trained for protection and combat. Basic dogs have 1+1 HD and are AC 7. Guard dogs have 2+2 HD and are AC 6. Dogs bite for 1d4 damage, guard dogs for 2d4. Both kinds of dogs have a base move of 15 and use the same experience table. Guard dogs will enter dungeons, but basic dogs won't unless they have the Fearless quirk.
There are several kinds of horses that are of interest to adventurers. Riding horses are lighter and faster than their more martial cousins, war horses, while draft horses and ponies are bred for labor. Horses are not as sure-footed as mules. They won't enter dungeons, and move more slowly through rough terrain. Ponies are sure-footed, but they won't enter dungeons either. The Horse Breeds table summarizes the attributes of all horses.
That rounds out the typical animals PCs take on their adventures. I hope you'll try this stuff out in your next game an let me know how it plays.
Dog photo by Flickr user MJ Klaver: https://flic.kr/p/8F4XSJ