These were once friendly Doberman Pinschers at some point in time, but the virus caused them to transform into vicious Man-Eating Hellhounds.
Their main way of attacking is a leaping bite, which causes moderate damage. On several occasions, they will make a surprising entrance by breaking through windows. The creatures make an appearance in every game up to Resident Evil 4, but their appearances are much rarer in later installments.
The monsters are very fast and can do a good deal of damage, so the best thing to do is kill them as soon as possible. Using the handgun will normally make quick work of them, but using the shotgun wouldn't be out of order when in a pinch.