Feral dogs in Greece are not a new phenomenon at all, but the population exploded with the economic crisis, when people couldn't afford caring for their pets (applies to cats as well).
This is fairly recent, and can be seen quite easily when you look at them more closely:
They look like breed dogs, with varying patterns, morphologies and colours, compared to dogs which have undergone feralization, and become yellow, slim and pointy-nosed.
More on feralization: https://www.sciencedirect.com/science/article/pii/S0169534719302307
There are still a lot of abandoned pets nowadays, mainly because of hunters and shepherds discarding animals that aren't fit to work anymore.
If you want to read more on the subject:
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