I'm not trying to make a joke, nor am I being facetious. Sorry, internet posts sometimes come across in a different 'tone' than I mean them to.

I'm really just trying to get down to the crux of what the lore is on trews. Some folks say that they were only used when riding, while I tend to think they were fashionable on their own, even for fellows who didn't plan on riding a horse. Leggings were popular throughout Europe, and there was plenty of European influence in Scotland that could have made tartan trews a distinctly Scottish form of this manner of dress.

And yes, I am well aware of what a riding crop is. I own several horses and do a lot of riding. And I can say as an avid horseman that I would never consider using a walking stick as a crop. It's too heavy and unbalanced for that. Crops are very light and very small diameter, meant to be used as a tactile aid. Trying to keep a large diameter stick in your hand while handling the reins (and I'm assuming the Scots rode English-style with both hands on the reins instead of one-handed 'cowboy style' like we do here) would be clumsy and uncouth for a gentleman.

I'll add the disclaimer that I'm not professing to be an expert on Scottish horsemanship by any means. But I do find the idea of using a walking stick as a riding crop to be very unlikely, especially considering the history and conventions of English (and pretty much all European) riding styles.

All that is to say that I would guess the fellow in the portrait is probably just on foot and choosing to wear trews instead of a kilt.