Both bullshit and horseshit are used to mean nonsense or waffle, especially to cover up a lack of knowledge or unwillingness to inform.
An old answer here describes them as near synonyms. If they're not exact synonyms, what's the difference in meaning or usage?
While bullshit may be shortened to bull, I've never seen horse used to mean nonsense. Perhaps this is just that horseshit isn't common enough to be obvious when the shit is elided. If the horse variant is just variation for added colour, are there others in regular use with very similar meanings? They're certainly not common if so.
In British English, bullshit, as well as being a noun, can be a verb (and occasionally an adjective as in a bullshit excuse). Horseshit on the other hand only appears as a noun (I inadvertently gave an example in the comments).
By coincidence, the blog Strong Language posted "a matrix of shits" since I wrote this. It doesn't answer my question, but does illustrate it.