It is possible to customize the
pfSense Captive Portal login page so a custom page will be shown. In order to do this, follow the steps below. This article assumes you already have the captive portal functionality configured and working and just want to change the page that is shown when users access your site. For more information on how to set the captive portal up, check out this video tutorial. It is from an older pfSense version, but the main steps are still the same.
simple sample customized login page allowing both an username/password combination and a voucher can be downloaded here
and will look like:
Another simple example which can be used in conjunction with free internet access (thus not allowing any way of authentication):
pfSense up to 2.2pfSense 2.3 and up
Both are optimized for mobile device displaying. You can easily use it as a starter and customize it to your own liking.
Thanks to Alessandro Agazzi for making me aware of the required changes for pfSense 2.3!