Card evolutions
Retrained is the term used to describe a monster that has been reprinted with a new effect and slightly different name, but with the same base ATK and DEF, and usually Level as well. Examples include "Battle Ox" to "Enraged Battle Ox" and "Magician of Black Chaos" to "Dark Magician of Chaos". Special cases are Harpie Lady 1, Harpie Lady 2, and Harpie Lady 3, which all have the same stats as the original Harpie Lady, but are not all representing the same creature. Rather, they are the three sisters from Harpie Lady Sisters.
The term "retrained" comes from "Retrained Elf Swordsman," a translation of the Japanese name for "Obnoxious Celtic Guard", who is a retrained version of "Elf Swordsman" ("Celtic Guardian").
Retrained monsters are not to be confused with upgraded ones, such as Megacannon Soldier. These, like retrained monsters, are clearly based on existing cards. However, unlike retrained versions, these have higher ATK and/or DEF and an effect meant to improve upon the original.