Casas rurales — self-catering countryside houses, the Spanish equivalent of a French gîte or a UK cottage rental — are a deeply established part of Spanish rural tourism, with large listing platforms and regional tourism boards promoting them specifically for walking and nature-based holidays. Many of these properties are standalone rather than shared with other guests, which tends to make them a lower-stress overnight option for a dog that struggles with hotel corridors or other animals nearby.
As with hotels, whether a specific casa rural accepts pets, and whether it charges extra, is set by the individual property rather than any single national network, though the major booking platforms for rural tourism in Spain typically let you filter listings specifically for pet acceptance. Under Spain's animal welfare law, the same signage and written-policy requirement that applies to hotels applies here too, so a clear answer on fees and restrictions should be available before you book, not just assumed.
Rural houses in mountain or walking-focused regions — the Pyrenees, Picos de Europa, parts of Andalusia's interior — tend to be especially well suited to a dog-and-owner trip, both for the accommodation itself and for direct access to walking routes straight from the property, without the beach-season restrictions that apply to some coastal accommodation in peak summer.