A cat darted out under my inexperienced horse this past fall, actually colliding into his legs (it was watching the horse ahead of us). I landed on the gravel road and broke a rib. Thankfully, my helmet-less head barely kissed the ground - I couldn't BELIEVE I'd forgotten it!
Loose dog off its own property is always fair game. Warn the neighbour that your safety is not an option and neither is his loose canines. Chase them down, bark at them with your biggest voice. Get animal control involved if this does not work.
Polite is NOT worth you becoming permanently paralysed from an accident.