I first came across the concept of a hoof pestle in a Lar Hothem artifact guidebook.
   Further research though, did not produce a substantial universal reference
   to an object known as a hoof pestle. Occasionally, internet auctions would
   reference a hoof shaped pestle, and once I found a reference to a hoof shaped
   pestle found in a cave in Israel. From what I can tell they appear in the
   Midwest in general; Ohio, Kentucky, Indiana and Missouri. They are associated
   with the Archaic age according these guidebooks, but they don’t seem
   to be associated with any chronology or certain people. 
  
Generally, a hoof pestle is modeled on some form of a hoof motif. The idea
    of a hoof shaped pestle can be subjective to some degree, but authenticating
    hoof pestles need not be completely subjective. Attributes to consider are:
    use wear consistent with grinding on the base area, adhesive use wear consistent
    with constant interaction of human hands and oils with the stone, and nicks
    and chips in the pestle for finger placement. The motif follows the similar
    pattern seen in Figure 1 and in the guidebooks.
  
    If you pick up a hoof pestle and rotate it, you will find a position fitting
      snugly between the thumb and first two fingers. The curved part of the
      pestle will go back toward the curve in the palm, and there may be little
      indentations in the rock where fingers fit. These At top: Figure 1. – A
      collection of hoof pestles Above: Figure 2. - Common components of hoof
      pestles are either through use or through the user making small chips or
      nicks in the rock or both. Intuitive feeling will usually allow the interpreter
      to feel a complete fit.
  
    After studying 26 hoof pestles from Missouri, I can reveal a few facts: they
      are commonly made of andesite, granite, quartz, flint or basaltic andesite;
      their average length is 2.5 inches, maximum length is 4.5 inches, and minimum
      is 1.97 inches. The average base grinding area is 4.75 (around) inches
      measured by clay impression. Average weight is 7.6 ounces with maximum
      being 20.7 and minimum being 2.9 ounces. Approximately 57% have a pocket
      the size of a small nut or corn kernel on the grinding surface.
  
    Components of hoof pestles, as shown in Figure 2, are small metates and anvils
      that could generally be carried anywhere and were mobile. It is generally
      easier to find hoof pestles than metates because metates were often turned
      over, and it would take time to wear a depression on the surface. It is
      obviously easier to identify a pestle than an unworn metate.
  
    
      |  | 
    
      | At top: Figure 1. – A collection of hoof pestles Above: Figure
        2. - Common components of hoof pestles | 
  
  
    Hoofs were used in a couple of unique ways in ancient America: (From Catlin:33), “Many
    of them also ride with a lance of twelve or fourteen feet in length, with
    a blade of polished steel; and all of them, (as a protection for their vital
    parts), with a shield or arrow fender made of the skin of the buffalo neck,
    which has been smoked, and hardened with glue extracted from the hoofs.” “The
    feet of the animals (buffalo) are boiled with their hoofs, for the glue they
    contain, for fastening arrow points and many other uses.” (Catlin:
    262). Perhaps the pestles were symbolic of the shield and the glue that held
    the tribe together.
  
    General conclusions: the range in size of pestles is indicative of the range
      in size of hands. I believe that there was wide scale use of children,
      teens, young ladies and older ladies in the production of foodstuffs, nut
      processing, and grinding. Portability is suggested in the size of most
      pestles and platforms. The abundance of hammerstones nearby may suggest
      nut cracking. This idea is congruent with the end of the last glacial age,
      the encroachment of deciduous forests, and beginning of the Archaic period
      in which large scale nut processing probably began.
  
    Why didn’t the hoof pestle survive? The rolling pin has survived for
    centuries: roller pestle to rolling pin, but not the hoof pestle. The cup
    stone, the axe and grinding stone all have their modern day equivalent. We
    do not have a metate anymore but we do have a cutting board. We don’t
    have a mano anymore but we do have meat tenderizers. Somewhere in a factory
    we now prepare grains on a much more sophisticated level.
  
    Did the hoof pestle disappear with the people that created them or did they
      find something better? To me, it seems logical that over time they developed
      pestles that were larger so they could use their upper arms more, and they
      also wandered less as subsistence farming took root.
  References:
    Hothem, Lar
    2000 Indian Artifacts of the Midwest, Book V, Collector Books
    Catlin, George
    1844 Letters and Notes on the Mannes, Customs, and Conditions of North American
    Indians, Volume I, Dover Publications(1973 reprint)