First, it must be clarified that at the time of preparing a will it is not necessary to prepare a list of assets, unless the testator wants to leave some particular asset to someone, this is called a legacy.
In the case of a pet, then, it could be assumed that what is left is a legacy; However, there is an obstacle and that is that, except in Mexico City, animals are not recognized as sentient beings, which makes it impossible for them to be considered to receive the goods of someone who has died.
Who can I choose as heir?
To any person, whether related or not, as long as they have the capacity to inherit in accordance with the law.
It is convenient to clarify that, regardless of whether the testator designates his heirs freely, it must be taken into account that, if there are economic dependents in his charge (minor children, incapable children, ascendants who lack resources, siblings, etc.), they must allocate part of their estate to comply with child support obligations, since otherwise this right can be claimed by the beneficiaries even when a will has been granted to someone else.
What if I want to ensure the welfare of my pet if I die?
A person must necessarily be selected to be the recipient of the goods, but these can be conditioned to taking care of your pet.