Dog packs come in many different models and designs, but they have several elements in common. Examining a pack for the points listed below will give you a good idea of the quality and usefulness of the pack you are considering.
Panniers.
These are the bags that hang on each side of the dog to haul your gear.
Roughly rectangular in shape, they are sometimes tapered for a more
ergonomic fit and are usually made from nylon. Some will have
additional features such as storm flaps over the zippers to repel
water, compression straps to secure and stabilize the load, or interior
pockets to help organize gear.
Belly and chest straps.
These nylon-webbing belts secure the pack onto your dog. Most
designs have a strap that wraps around your dog’s chest and one or two
belly straps that secure the panniers underneath your dog. Look for
wide straps (narrow ones can be less comfortable) and some kind of
padding over the straps (often a fleece or closed-cell foam wrap). Make
sure the buckles are sturdy and lie flat so they don’t dig into the dog
as the pack moves.
Backpiece.
This is the connector between the panniers that rides on your
dog’s back. It is a critical component of the pack, because the weight
of the load will be focused here. On some designs, the backpiece is a
solid piece of nylon (often padded), while other models have two or
three nylon straps. Each style has advantages. Solid backpieces
distribute the weight better and are often equipped with mesh pockets
or bungees to hold additional gear, or a handle to help carry the pack
when it’s not on your dog. Models with straps adjust to mold the pack
more closely to your dog’s contours, ensuring a more natural fit.
Other goodies.
Packs may other include other features and conveniences as well. Compression straps help to stabilize the load and can be used to lash gear on the outside of the pannier. Exterior pockets are great for items that need to be accessed quickly (like poop-scooping bags). Some packs even include integrated water bladders!