Most Spaniards I teach are relatively well off. Talking to them I don't get the idea that many of them have "servants". If the kids need parking somewhere before they are old enough for nursery, kindergarten or school or after they come home from them it tends to be with the grandparents rather than with child minders. Cleaners are pretty common but they don't seem to do the ironing or the laundry. My perception is that, unlike middle class Britons, there is not a culture of buying people's labour to give the better off middle classes more free time.
So a dog walker is out of the ordinary. Of course it may well be someone just having a go, they may not really have a business, but the fact that they are having a go at all in cash strapped Spain is a sign that they think there is a market.
27€ for twice a day, seven days a week or 50€ a month if you're interested and they'll do the vet runs and suchlike too.