A cohesion of sheep! Perhaps not a practical term to use to describe farm animals (I feel like "cohesion" would be too much of a scientific term to use regarding sheep) but an accurate description, nonetheless. After all, the property of cohesion is defined as the ability of polar molecules to "stick" to other polar molecules - and cohesive groups of people are those who stick together.

Therefor, a flock of sheep should be a cohesion of sheep.

My new word for a flock of sheep is:

A 'Painintheass' of Sheep.

Because that's what they are as you're happily driving down some picturesque rural road, and the road is only wide enough for one vehicle, and you're happily plodding along at a quite slow pace, then, as you round a corner you're confronted by three hundred sheep being herded from one field to another.

A Painintheass of sheep.