Enterprise NX-01 encountered a pod while investigating a a convergence of several anomalies in the Delphic Expanse in 2153. (ENT: "Harbinger") While Enterprise tried to use the grappler to remove the pod from within the anomalies, the anomalies expanded around the ship. The pod was eventually removed and brought on board Enterprise.
The pod is believed to allow an individual's bio-signs to be recorded to gauge the effectiveness of the Sphere Builder's terraforming of the expanse, as numerous electrodes were hooked up to a test subject.
After the disappearance of the individual inside, the pod remained on Enterprise. A hologram of the pod and its subject was later used by Jonathan Archer as evidence to try and convince the Xindi Council of the Sphere Builders' real intentions. (ENT: "The Council")