The word "biscornu" is, indeed, a regular French word and is correctly
spelled. However it is, officially, an adjective and not a noun.
According to the Dictionary of the French Academy
), the adjective "biscornu" originates in XVI century, composed by the
prefix bis- which gives idea of plural, and "cornu" = "with horns"
(literally it would be, "several horns") and means: having an
irregular shape, bizarre, unnecessarily complicated.
Familiarly -- not still officially -- the adjective has acquired a
noun-like use designating the eight-vertices tiny cushion you
described in your question. Although it is typically used as a
pincushion, please mind that the french word for pincushion is
"pique-aiguilles". It's easy to infer that the use has turned into a
noun-like one by dropping the originally associated real noun:
"(coussin or coussinet) biscornu", meaning "irregular (cushion or
"Coussin 'biscornu'" (http://galeries.nah-ko.org/album02?page=2 )
"...ce petit trucmuche c'est un biscornu, un petit coussin cousu d'une
certaine façon qui lui donne cet aspect "biscornu" d'où son nom..."
(http://blogs.aol.fr/isanoma/undeuxcroix/entries/1091 ) "...this
little doodad is a biscornu, a little cushion sewn in a certain way
that gives it that irregular aspect, hence its name..."
I guess this answer satisfies your requirements, otherwise please let
me know by posting a request for clarification.
biscornu (Google Image search)