No, Im with other Steve on this - its for vehicles used on the road and is calculated by emissions. Its not just vehicles or BTCC cars would have to pay it, its not just emissions or power stations would have to pay it.
As for horses vs dogs, its entirely down to the health aspect, no-one has a problem with horses shitting in fields?
Are people just being pedantic for the sake of it - stop and think before you type.
Ummm, they do pay tax don't they?