Unnatural Twenty

Uncommon companions include animals that are not commonly found among humanoids, such as a snake, wolf, hawk, and monkey. Wolf Companion, uncommon (requires attunement) Wolves are pack animals, and they adapt easily to fighting alongside groups of humanoids. You have advantage on melee weapon attack rolls when an ally is adjacent to your target. Hawk Companion, uncommon (requires attunement) Hawks hunt by eyesight over long ranges. You have advantage on Wisdom (perception) checks that rely on eyesight. Monkey Companion, uncommon Monkeys generally live in large groups, and survive by sharing food whenever possible. You may make a Dexterity (sleight of hand) check to take an item within 50’, if that item weighs less than a monkey. Viper Companion, uncommon (requires attunement) Vipers are small venomous snakes. When you do piercing damage to a creature with a melee weapon attack, you may use a bonus action to inflict an additional 1d6 poison damage....

Companions are like magic items from a mechanical point of view. A character finds one, attunes to it, and gets some kind of benefit. Beyond that it’s just assumed that you have a dog or whatever following you around, but it’s not a major character in the story. If it were, then it would be a PC (which I’ll cover later). Asking your dog to run up and attack a demon is not part of this design. Also a companion is not a familiar, and does not gain any magical abilities beyond what a normal animal of its type could do. Companions can be used as an alternative to magic items, for example in a low magic campaign, or alongside magic items. If one character finds a magic shield, and another gains a wolf companion, they both gained some benefit. Companions have rarity, just like magic items, including common, uncommon, rare, very rare, and legendary. Common companions include normal animals, such as a cat, dog, mouse, or sparrow. Sheepdog Companion (dog), common A sheepdog is ...