undermounted

adjective2

1. •
   Something that is mounted under something else.

   • The only engine having a design adapted for heavy traction work. The undermounted construction gives it many points of important advantage over the top mounted style as used by all others. — 1909
2. •
   Said of a rider mounted on a horse that is not strong or large enough for the size and weight of the rider.

   • “It's a pity to see a gentleman undermounted,” specially in a country like this: so dangerous too! — 1899