The 2.5" long "thing" shown below was found under decayed straw around a potato plant in the garden.  This is a hornworm pupa, the intermediate, transformation stage of a tomato or tobacco hornworm on its way to becoming a moth.  The hornworm caterpillars drop from infested tomato (or potato) plants, burrow into the litter or the soil and spend the winter as pupae. The moths emerge in the spring. No control is necessary. The caterpillars have already fed on the leaves and no further damage will occur this season.  See our ISU Insect Information Note for more information.
You can discard pupae as they are discovered during digging and tilling, or if you would like to see the adult moth you can keep any pupa in a container buried in a few inches of dirt in an unheated garage or porch for the winter. Next year transfer the pupa into a bit larger container with a stick for the emerging moth to climb up on so it can expand its wings properly.  The moth should emerge in late May or June.  Once the wings are fully expanded you can release the moth. 
 
 
Tomato Hornworm Pupa. Photo by D. Bangert.