Looks like a wolf to me
I don't think it's my imagination but I reckon I can see pointy ears, mastines' are droopy.
Also it's not tall enough to be a mastín, they have long legs.
From the description of it's behaviour I also rule out an alsation type dog which would be more likely to be running amok instead of stealthily stalking.
Check out the photo from Carlos Sanz re "bulky", nothing skinny about his wolf.
Shame it's stalking unprotected domestic animals instead of rebecos.
The National Park recently (June) handed out licences to shoot 4 wolves, I think this is a lucky one.
There's a small pack of wolves that tend to hang around the eastern massif. Sotres is in the middle of the central and eastern.