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i guess it could be related (Thayers > Thayes > Thays), but i've found another possibility.

There is also the surname Thaysen and its variants Theysen and Theisen (and many more). These surnames mean 'son of Thays' (or Theys or Theis) and Theis/Thijs is from Matthijs, the Dutch form of Matthew (and Theys from Mattheys, both Dutch and German form of Matthew). And although the more common German form of Matthew is Matthias, i still found quite some Matthais at Google.com, especially present in Germany. So, this might be another possibility, though you might wonder how it got in Spain, but this is the same problem with the Thayer/Thayes-possibility.