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
git clone -b stone_soup-0.17 https://github.com/crawl/crawl/ crawl-ref
# cd crawl-ref && git checkout stone_soup-0.17 && cd ..
make