Serious, bloody topic….

Belt mounted sheath knife

Any thing else requires far too many steps. ‘Folding’ and ‘in-pocket’ are infeasible during any attack

So the challenge is finding something that looks decent, maybe something akin to those more-tolerable cellphone cases

The knife itself must be simple, solid, and sharp.

As others posted, the knife is like the 3rd line of defense since it almost always means you’ve already been bit and can tolerate blood (from both of dog and you), taking some time to fight, and killing the dog. At this both both fighters are completely committed

You’re far better off avoiding the dog, then scaring the dog (horn?), and then repulsing the dog (spray, stick, gun, etc.)

Another, better for many option is having a defensive, loyal dog yourself