I stick with numbers, even on my long-time favorite cows. When my out of town grandkids come to visit though they seem too want to name them.  Riding around in the Ranger a couple years ago, I pointed out my cow #66 to my then 10 year old grand daughter. She says back to me very matter-of-factly, "Grandpa, that's not #66, that's Molly". 

So forever after, #66 became Molly!