These formulae are unversioned (they have no stable release) so are only available as the latest version from e.g. a version control system.

There are a few problems with head-only software:

• Since there's no stable release, the software is liable to change at any moment. We can't guarantee that head-only software will build reliably or be compatible with other software which builds against it, and that increases our support burden.
• Homebrew's version management and upgrade mechanisms don't work with head-only software.

Contact the software's developer and ask them to tag a stable release. If the software has a stable release and works reliably, feel free to submit a pull request at the main Homebrew repository.

brew install --HEAD homebrew/head-only/<formula>

Or brew tap homebrew/head-only and then brew install --HEAD <formula>

Or install via URL (which will not receive updates):

brew install --HEAD https://raw.githubusercontent.com/Homebrew/homebrew-head-only/master/<formula>.rb

brew help, man brew or check Homebrew's documentation.