Carrier

Carrier is creature type originally unique to Urza's Legacy. It first existed as a vertical cycle of four black Phyrexian creatures which spread disease. Otherwise they could have been just Horrors.

The Cycle
Each of these creatures can be sacrificed to give another creature -X/-X until end of turn, where X is its power and toughness.
 * Phyrexian Denouncer (1/1)
 * Phyrexian Debaser (2/2)
 * Phyrexian Defiler (3/3)
 * Phyrexian Plaguelord (4/4)

Note

 * The white card Carrier Pigeons doesn't have the subtype Carrier.
 * More surprising, the black Disease Carriers from the set preciding Urza's Legacy (Urza's Destiny), with similar design, hasn't the Carrier-type either.
 * The same can be remarked concerning Maggot Carrier from Planeshift.
 * It would not be until Modern Horizons that another card with the subtype was seen, this being Plague Engineer.