We have mentioned stories or incidents involving emus, dogs, rabbits, wild pigs, turtles, deer, a bull and even cat fish running, hopping or slithering across runways, but incidents involving cows seem to be greater than one might think.
For starters, in an August 2010 US National Transportation Safety Board (NTSB) accident report excerpted on General Aviation News, a Cessna 182 pilot in Del Rio (Texas) was attempting to land on a private dirt strip. However, he became distracted by a cow adjacent to and moving toward the runway (being Texas, the runway must have been on a big cattle ranch) – 
Granted, that was in Texas. However, NYC Aviation reported early last year an incident at the Mayor Buenaventura Vivas Airport in Santo Domingo, Venezuela, where an Aserca Airlines McDonnell Douglas MD-82 struck two cows (who died instantly) because they did not move fast enough. The jet’s left main landing gear and left wing flaps were damaged enough to keep the aircraft grounded until repairs could be made.
NYC Aviation also mentioned that in 2005, a herd of cows were hit by an Air France Airbus A330 while landing in Port Harcourt, Nigeria. That led to local authorities arresting stray cows and holding them until their owners paid a fine. In addition and in 2011, an Aviastar Twin Otter hit three cows while landing at the Komodo Airport in Indonesia (at least they did not hit any Komodo Dragons!)
However and if you think cows only pose a threat to aircraft in Texas or in developing countries, a British biplane clipped one while making an emergency landing in a field that was caught on video back in 2008:
Luckily, neither the cow nor the aircraft were seriously injured and the pilot was glad to avoid hitting a fence that could have led to worst damage.
In other words, keep an eye out for any wayward cows when landing on runways – especially grass runways or fields.