A Sphere-Builder transport pod
The transport pod was a type of spacecraft that allowed a single individual from the Sphere-Builders' trans-dimensional realm to be monitored in terraformed sections of the Delphic Expanse. The pod was believed to allow an individual's bio-signs to be recorded to gauge the effectiveness of the Sphere-Builder's terraforming of the Delphic Expanse, as numerous electrodes were hooked up to a test subject.
Enterprise NX-01 encountered a pod while investigating a trans-dimensional disturbance in the Delphic Expanse in 2153. While Enterprise tried to use its own grappler to remove the pod from within the anomalies, the disturbance expanded around the ship. The pod was eventually removed and brought on board Enterprise. (ENT: "Harbinger")
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")
In the final draft script of "Harbinger", this type of craft was described as "small". Its top half was referred to as opening automatically. In the equivalent version of the scene from the final version of the episode, however, a portion of the craft is manually opened instead.