I tried put some healing potions in pet's inventory, inside "Pet Consumables" section, but it seems pet does not use them. Is it possible to heal your companion pet when it has low health?
Pressing Shift+Z will give your companion pet a health potion.
It is not possible to give your pet a potion when already at full health.
Giving your pet a potion will place a temporary indicator next to their health frame showing how much time is left on the currently active potion;
It is also possible to give your pet a mana potion by pressing Shift+X
There are two ways of healing your pet with potions.
First one is by using Shift+Z to give them the best HP potion you have, or Shift+X to give them the best mana potion you have (you can also change these shortcuts in Options>Controls).
Second way of healing your pet is to simply put a mouse cursor over it's HP or MP bar and left-click once.