Do people use this for cows? If so, why is it so hard to undo?

TonyE may correct me... Cow hitch is the type of knot and this one would be used for creating a framework. Being difficult to untie would mean that if you tied a horizontal stringer between two poles, it would be unlikely to fall off.

This knot is known by many names - , Lark's head, Lark's foot, Girth hitch, Ring hitch, Lanyard hitch, Bale Sling hitch, Baggage Tag Loop, Tag Knot, Deadeye hitch, Running eye. It is normally used to attach a rope to an object and is easy to untie. When used in this configuration, however with the cord ends going through the bights of the knot it is difficult to untie

The cow hitch is known by many names - See my reply to bingley. Its effectiveness in your situation would depend on the width of the stringer and poles, the cordage used, how tight you make the initial hitch and frapping turns. Try it out on one pole to see if it works for you. Google square lashing to see other examples

Thanks, Tony!

Thank you, Tony!