As of 0.18, monster is part of the crawl source distribution:

  https://github.com/crawl/crawl/

in directory crawl-ref/source/util/monster/

To build monster for 0.17 (likewise 0.9 through 0.16):
  git checkout dcss017
  # On the first build:
    git clone -b stone_soup-0.17 https://github.com/crawl/crawl/ crawl-ref
  # Subsequent builds:
    # cd crawl-ref && git checkout stone_soup-0.17 && cd ..
  make