To expand on the idea of pets (they can be attacked and killed), how about a?
Rat - it can sneak around and scout, also fit in small holes and get small treasure.
Donkey - it carries extra weight.
Snake - it bites and causes the poisoned condition on a successful bite.
Song Bird - once a Short Rest it grants Inspiration to one person of your choice.
Chicken - once a Long Rest it lays an egg that heals 1d4+1 damage, and on rare occasions a golden egg that heals 8d4+8 damage or is valued at 100 gold.
Dog - it can be used to gain Advantage on Investigation & Perception checks, as well as be an attacking meat shield in combat.
What would you add?
