The tax on vehicles or animals shall be leviable upon the owners of or the person having possession or control of such vehicle or animals in respect of which the tax is leviable :

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Provided that in the case of an animal generally used or employed in drawing any vehicle, the tax in respect of such animal, shall be leviable upon the owner of, or the person having possession or control of, such vehicle, whether or not such animal is owned by such owner or person.