Never thought of that before (その発想はなかった) is a tag for artworks describing a fresh, new and sometimes blizzare concept or idea potrayed in artworks, that did not exist before prior to this. However, It won't be surprising to see rehashed concepts and copied memes thown into any works either.
The origin of this tag seems to be originated from a variety show called "Downtown Gaki no Tsukai Ya Arahende!!", aired on Nihon TV, where a character name Itao's Wife has this favorite phrase saying "Sono hassou wa nakatta wa" (note the "wa" which is used for feminine speech in Japanese). In English, this translates to "never thought of that before~!" (the ~ is used to add a feminine tone in a sentence). This tag itself is a derivation from the above sentence.
Although mainly used for meme illustrations, it is a tag with praises given to illustrations with extraordinary creativity or ideas that one would most likely never thought of it until it exists.
↑The very first Never Thought of That Before illustration, circa 2007.
Winning Concept Thought of That Before No Need For That Concept Shouldn't Have Thought of That No Such Concept I'll Be Damned If That Concept Exists None Other Concept Than That
As well, there are quite a number of cases where "this" is used instead to "that".
Never Thought of This Before Thought of This Before No Need For This Concept Shouldn't Have Thought of This No Such Concept I'll Be Damned If This Concept Exists None Other Concept Than This
