calypto-, calypt-

(Greek: covered, cover; hide, hidden; conceal, concealed)

eucalyptus (s) (noun), eucalypti (pl)
Any of numerous often tall trees belonging to the genus Eucalyptus, of the myrtle family, native to Australia and adjacent islands, having aromatic evergreen leaves that are the source of medicinal oils and heavy wood used as timber.
Eucalyptus (s) (noun), Eucalyptuses or Eucalypti (pl)
1. Any of numerous tall trees of the genus Eucalyptus, native to Australia and having aromatic leaves that yield an oil used medicinally and wood valued as timber.
2. Etymology: from Modern Latin, coined in 1788 by French botanist Charles Louis L'héritier de Brutelle (1746-1800) from Greek eu-, "well" + kalyptos, "covered", with reference to the coverings on the buds.
Four Horsemen of the Apocalypse (noun) (no plural)
A compound word that refers to war, famine, pestilence or conquest, and death: The Four Horsemen are personified as the four major "plagues" or "evils" of mankind; which will come at the end of the world.

The Four Horsemen are used to describe people or other agents that can result in imminent catastrophe.