Frakir is the name used for more-or-less this concept in the _Amber_
  series by Roger Zelazny.  `She' appears in the later books in the
  series, with Merlin (bad name, I know :-).  She's a creation of the
  Courts of Chaos, and Doesn't Deal Well (TM) with extreme
  representations of order (in the story, the Pattern of Amber and the
  Unicorn both cause her to react as if Merlin is in danger.  She is
  completely inactive during his trips through the Pattern (symbol of
  order)).

  Frakir is a semi-intelligent strangling cord, capable of independant
  motion and mild telepathic reception.  Frakir also possess a form of
  `sixth sense', capable of detecting certain types of danger and
  hostility.  Frakir's telepathic abilities do not extend in the other
  direction - she can understand mental commands sent by her bond
  holder from up to about 100 meters away, but she cannot
  telepathically communicate in the other direction.  Her normal
  method of communication is to `pulse' or constrict lightly around
  the arm, leg, or other appendage.  She knows a small number of
  pulse-signals, and can project her emotions roughly via an extremely
  low-level empathy.

  Frakir appears to be a black and purple silken cord about .8 meters
  long with occasional silver threads running throughout. (Merlin's
  colors) Under normal conditions Frakir will not wear or fray - see
  below.  Normal mode of operation is for frakir to appear as a woven
  bracelet, anklet, or armband, but other variants are possible.

  For game-mechanical purposes, frakir is a cross between a familiar
  and an enchanted item.  I have some ideas about how creating one
  might go, if you care (variant on find familiar, used with an
  enchanted strangling cord and a living snake to sacrifice).  during
  `bonding', frakir takes hit points from her companion, as with _find
  familiar_.  Destruction of frakir follows roughly as with _find
  familiar_.