The Ruff Wear Stow’n Go Leash
In case on emergency, you should always have a spare collar and leash for your dog with you.
Maybe, your dog has been running off leash but has gotten injured.  The first thing you need to do is restrain him.  You have to use whatever you have handy.  With the Stow’n Go Leash - you have everything you need - collar, leash, and pick up bags in a compact case. 
Features:

* The integrated collar fits any size neck. 

* The ultra light leash weighs less than 2.5 oz (79g). Perfect for backpacking.
Below the leash handle, there is another small loop where you can attach the case.
The main part of the leash is so thin that you could even use it as an emergency muzzle. (I tried  it on Tucker.  You  have to fasten it so the collar buckle  is out of the way, but it worked.) 

*  The leash  rolls up to fit inside a case about the size of a sunglass case, made of the quality you would expect of a Ruff Wear product.  The case has a small mesh pocket inside for pick-up bags, and an opening on the outside to pull the bags through. (The fit is a little tight, but I guess that’s better than having the bags fall out. I find it just as easy to simply unzip the case and get a bag out that way.)  On the back of the case, there is a loop stitched at both ends, and a loop with velcro, to provide many ways to fasten the leash to the case and fasten the case to a belt or pack.  The whole case is certainly small enough and light enough that your dog could carry it when he wears a pack (like the Ruff Wear Approach Pack).  

Bottom Line: A great spare/emergency collar and leash.