Arnold for Maya is included with a default install of Maya via the mtoa.mll plug-in. This plug-in is auto-loaded, and Arnold is set as the preferred renderer in Maya.
In the case where Arnold is not loaded by default, you can manually load it via the Plug-in Manager (Windows > Settings/Preferences > Plug-in Manager) by enabling mtoa.mll.
A mental ray to Arnold transition guide is available. See also the Arnold for Maya User Guide.
Install Maya without installing Arnold
You can choose to opt out of the Arnold installation by disabling the Arnold for Maya option during the installation process.
Use existing Arnold licenses alongside Arnold for Maya
Pre-existing Arnold licenses are only consumed in situations where you need to render without a watermark; for example, during batch rendering. They are not consumed for interactive rendering with Maya.
To convert existing assets from mental ray for Maya to Arnold for Maya
A sample mrShadersToArnold.py conversion script is available in the /docs directory of your mtoa installation folder.
Difference between Arnold for Maya and Arnold
This local version of Arnold for Maya only works interactively within Maya. You can render a single frame or a sequence of frames interactively; however, batch rendering and calling from mayabatch.exe are not available and your images will appear with a watermark. To render images without a watermark, purchase Arnold batch render nodes from Solid Angle.