Dracula X: Rondo of Blood
Castlevania: Order of Ecclesia
Skull Spiders appear in various areas of Order of Ecclesia. One appears in the sunken ship area of the Kalidus Channel area, and many appear in the Tymeo Mountains area. They are highly annoying, for touching them will poison you, but also very slow, allowing you to defeat them easily. However, in Hard Mode, they move twice as fast, and become ever increasingly hard to hit. They drop a rather uncommon Glyph sometimes upon defeat, the Fidelis Aranea glyph. It allows you to summon a Skull Spider to attack for you. The Skull Spider will move faster as you level up the glyph.
Castlevania Judgment
The Skull Spider appears in this game, but this time as an accessory that can be equipped on the wrist. It can only be unlocked by linking the Wii to your DS if it has Order of Ecclesia in it via Wi-Fi.