I don't agree, unless they had some pretty massive animals :o)

you can keep a few animals in by building a fenced enclosure. There's no need for a ditch - a waste of effort - animals will quickly erode the sides and do themselves and injury in the process. A henge bank would similarly need a lot of maintenance to prevent it eroding to a point where animals can just stroll up it - sheep can climb hills steeper than you can make with loose earth. The design issues alone make a henge an impractical animal enclosure.

The other option to keep animal coralled is of course - trees.

I do think however, that some stone circles may be particulalrly ornate sheepfolds!