In my experience the most likely mammal you are likely to encounter in the river other than other anglers, are dogs! I have even been knocked of my feet by an over exuberant Labrador (thankfully towards the bank, avoiding a soaking). Around Dollar and Tillicoultry I have often come across mink scurrying about in the riverside trees and bushes, and occasionally in the river fishing. I know that there are otters on the river, I see their scat but have only caught momentary glimpses of them.
Mick Berney had much more luck this week and was able to snap this lad playing in the river:
This area has been a bit of a mink hotspot in the past. One of the good things about the otter is that they will not tolerate completion from mink and their presence will generally drive down the mink population.
Thanks to Mick for sharing the pictures