Doxygen isn’t installed by default. You can download it using brew.
$ brew info doxygen doxygen: stable 1.8.9.1 (bottled), HEAD http://www.doxygen.org/ /usr/local/Cellar/doxygen/1.8.9.1 (5 files, 15M) * Poured from bottle From: https://github.com/Homebrew/homebrew/blob/master/Library/Formula/doxygen.rb ==> Dependencies Optional: graphviz ✘ ==> Options --with-doxywizard Build GUI frontend with qt support. --with-graphviz Build with dot command support from Graphviz. --with-libclang Build with libclang support. --HEAD Install HEAD version $brew install doxygen