Bugs? Brush some onto some white paper and do some investigating. If they squish red, they're definitely bugs. Anyway, it's a lot easier to tell what it is, (bugs, dirt, etc.) once you have some on white paper.
Would you be able to post a picture of them? I know here sometimes you will find (usually only with horses kept in large pastures with tall grass) bott eggs which are yellow dots on horse's legs. I would assume those are similar, eggs of a bug that is in the pasture.
I would try to look up what they are but generally speaking would not be concerned.
When I had my horses on my property I found they were really difficult to remove.