The four-legged dog-like creatures from the first Borderlands return to Borderlands 2 and remain relatively unchanged. If you have played the previous entry, then you know that Skags have relatively thick skin that reduces your damage, but once they open their mouths and roar, you have found their weak spot. Skags aren't as common in Borderlands 2, though, as you will only find them in a few locations, but you still need to be careful, as they will be encountered in packs.
The weakest out of the whole species, Pup Skags are more of an annoyance than anything else, especially if you're trying to concentrate on the more dangerous Skags. They can leap at you, try to bite you and even use their tongues as a melee attack. You shouldn't have to worry too much about waiting for them to open their mouths to inflict major damage.
A potentially more dangerous Skag, Adult ones are meatier and inflict a lot more damage than Pups. In addition to the same attacks as above, it can also do a charge maneuver that can take off a lot of your health.
A ranged attacker, Spitter Skags will sit back and spit corrosive goo at you. Since they are usually always spitting at you, their mouths are more prone to be open for critical hits.
Barf Skags are similar to Spitters, although they will vomit up a corrosive goo when close enough, which can temporarily blind and slow you down. They can also perform a spitting attack when not that close that can still impair your vision.
If you thought other Skags were a problem, then you haven't dealt with the Alpha Skag yet. These heavily armored Skags can take a lot of punishment and deal a lot of it, as well. Both corrosive and incendiary elements are effective against them, so it's smart to bring those along when you know you might encounter one. Alpha Skags especially like to charge, so when you see it rearing back, get ready to dodge.
All Badass Skags in Borderlands 2 are elemental and some of the most dangerous enemies in the game. They have the melee attacks other Skags have, as well as shooting an elemental ball at you from a distance. In addition to that, they can pound the ground and send out a shockwave that corresponds to their element. The most dangerous aspect of Badass Skags is that they can transfer their elements to other Skags, so if you're fighting five others at the same time as a Badass Corrosive Skag, you could have six corrosive skags to contend with.