The undercloud used to install with Tuskar UI. It no longer does, as folks are now working on another GUI based on the REST APIs. So, for the time being, there is no Horizon on the undercloud.
Now, when you deploy the overcloud, that is a different history. After you do openstack deploy, and it goes through the installation, if you have a successful install, it will finalize with a message and an overcloudrc file where you can find the location of Horizon and the credentials for the overcloud environment.
LTG Federal, Inc