Canmore lists this as a 'possible' standing stone. It's a very curious looking thing - its general size and shape suggest a prehistoric standing stone, but in 1821 someone saw fit to attach a round sandstone ball to the top of it with an iron bar. It's 1.6m tall and is supposed to have been moved from wherever its original position was when someone wanted to widen the road.
In 1982 local folklore held that prior to the 19th century messings-about, newly wed couples strolled around it for good luck (in a sun-wise direction, I'd like to bet).
'Dagon' is actually the name of a Philistine god, who was half-man half-fish. But with a Scottish accent it no doubt derives from something much closer to home (assuming it's not just the romantic invention of an antiquary).
You can just about see the stone in the photo on this webpage. The page describes a walk to Loudoun Hill - if you visit Darvel you won't be able to miss this striking hill which dominates views to the east: an unusual landscape feature which must surely have caught the eye of our prehistoric ancestors. Check out the photo at the Sorbie Family's webpages: http://www.sorbie.net/hill_photo.htm