Web Spinners have been regular enemies in most of the Resident Evil games. As the name suggests, they are spiders that have been mutated via T-virus infection, which has resulted in tremendous size increase and aggression. Ironically, although they are called "Web Spinners", these creatures no longer produce webbing.
There are various types of Web Spinners produced from different species of spider. Some have been produced as B.O.W.'s, but others are Irregular mutants created by accidental viral exposure. Virtually all of the different Spinner types possess the same abilities and behaviour, attacking humans and other animals on sight by mauling them with their feelers or by spewing acidic venom from their maws that can leave victims poisoned.
The Umbrella Corporation developed the first Web Spinners at the Arklay Laboratory by injecting the T-virus into tarantulas. The results were unstable and the specimens proved to be uncontrollable, and so Umbrella abandoned the research. However, the T-virus outbreak occured before all of the subjects were disposed of and many escaped and were allowed to roam freely throughout the mansion and the adjoining residence.
When the S.T.A.R.S. fought against them, the giant tarantulas often released clutches of young spiders upon death. The babies would swarm and bite at potential prey, but proved to be little more than a nuisance. Of course, the Web Spinners could be prevented from birthing if killed using incendiary weaponry.