I am legend, has actually influeced the zombie genre. in the book, they were more like vampires I believe. a pandemic of some sort caused the people to mutate into the vampire things.
Last man on earth they were vampires. The Omega Man they were more like crazy mutants, maybe along the the lines of the Crazies (another Romero film). I am legend they were also more vampiric (and when you think about it, a possible sure for an horrible diseae like cancer, could potentially have side effects. how scary is that. no more cancer, but everyone becomes some undead thing?) In I am omega they are more like zombies.
In the will smith version the dogs name is Marley. The leson we learn from this is you have keeps your wits about you. You lose it you risk your dog dying. Now if we had a situation like this happen atleast we got Will frickin Smith on our side. He's a smarty pants doctor who will eventually come up with a cure, and he kicks butt, and he aint bad to watch while he's working out.