Mechanical Spider

A metal spider is an automaton made by Hephaestus. They constantly trigger Annabeth's and other children of Athena's arachnophobia.

The Lightning Thief
Percy, Annabeth, and Grover had to go to a ride in Waterland to retrieve Ares' Shield. There was a trap set by Hephaestus which involved metal spiders. Percy and Annabeth had to go through the tunnel in order to escape them. Annabeth, as a daughter of Athena, is naturally afraid of spiders.

The Battle of the Labyrinth
Eurytion gives Percy, Annabeth, Tyson, and Grover a metal spider so they can find Hephaestus. Hephaestus had given this spider to Eurytion as a favor as he had helped him pull a prank on Ares and Aphrodite. The spider was in the form of a medal with a depression in the center. When the depression was pressed, it changed into a spider. This particular metal spider could shoot metal web fiber to swing across bars.