See List of Spider-Slayers for a full breakdown of all versions of the Spider-Slayer.
The first series of robots were originally designed and built by Dr. Spencer Smythe with the financial backing of J. Jonah Jameson who piloted them for the pleasure of personally hunting the superhero he hated for capture. However, Spider-Man always managed to defeat each robot in turn. Smythe's continual efforts to perfect his machines backfired on him, leading to himself being fatally contaminated by radiation poisoning from the building materials of his creations. Blaming both Jameson and Spider-Man for this, he attempted, as a final act, to murder them both, but died just before Spider-Man thwarted the attempt.
Later, Jameson commissioned another scientist, Dr. Marla Madison, to create a new and improved Spider-Slayer. While that attempt failed, he eventually fell in love with her and married her (Amazing Spider-Man #167-168).
In Amazing Spider-Man Annual #19 (1985), Smythe's son, Alistair, emerged as the new builder of Spider-Slayers. He swore revenge on Spider-Man, repeatedly attacking the superhero with his own series of Slayers. Smythe later mutated into a humanoid Spider-Slayer, but remains a minor foe.
Interestingly, the original Spider-Slayer was seen among the robots and machines in the Reanimator's collection. It was later used by J. Jonah Jameson to attack the She-Hulk after she had married his son John, but it was destroyed. Wolverine later destroyed the Spider-Slayer when the Reanimator attempted to use it against him.
In issue #100, Nick Fury surrounds Peter's house with them in an attempt to arrest Peter. This is foiled by the arrival of the Ultimate Fantastic Four.
Here, the Spider-Slayers are about eight feet tall. They balance on a large sphere, with another small sphere for their heads. They wield two arm-cannons. They are not manned, but remotely controlled by the Ultimate Tinkerer.
The robot's pulse beams are enough to knock out the malevolent, super-powered entity Carnage.
The 1994 Spider-Man Animated Series had the Slayers being commissioned by Norman Osborn for hunting Spider-Man. This culminated in Alistair Smythe working for the Kingpin; he would go on to not only recreate the "Black Widow" Spider-Slayer, but also at the same time to unleash the "Tarantula" and "Scorpion" Spider-Slayers. In this depiction, the three Spider-Slayers could join together as a massive engine of destruction.
Later, Alistair would build another Slayer called the "Mega Slayer", a heavily-armed humanoid robot operated by remote.
Alistair left the Kingpin's service after, as punishment for repeated failures, the Kingpin's new chief scientist, the mutate Herbert Landon, genetically mutated him into the "Ultimate Spider-Slayer". In this form, Alistair was capable of walking again and was incredibly strong and resilient. He also had twin horns growing from his shoulders, both of which were organic laser guns.