Wikipedia supports both "old dog" and "pig dog" as possibilities:
"According to Pokorny[37] the English name "greyhound" does not mean "gray dog/hound", but simply "fair dog". Subsequent words have been derived from the Proto-Indo-European root *g'her- 'shine, twinkle': English gray, Old High German gris 'grey, old', Old Icelandic griss 'piglet, pig', Old Icelandic gryja 'to dawn', gryjandi 'morning twilight', Old Irish grian 'sun', Old Church Slavonic zorja 'morning twilight, brightness'. The common sense of these words is 'to shine; bright'." (http://en.wikipedia.org/wiki/Greyhound#Histo
ry,
penultimate paragraph)