A tiny freshwater fish, Poecilia reticulata, popular in home aquariums, that usually has a plain body and black or dark blue tail for the females and a more colorful tail for the males.
Fred kept a tank of guppies by the windowsill.
By extension, any tiny fish.
A tube holding paintballs before they are loaded into the gun.
2001, John R. Little, Curtis Wong, Ultimate Paintball Field Guide (page 177)