plymouth (graphical boot animation and logger)

Plymouth is an application that runs very early in the boot process
(even before the root filesystem is mounted!) that provides a 
graphical boot animation while the boot process happens in the 
background.

For instructions how to install and configure Plymouth and themes on
Slackware, see this topic on linuxquestions.org

https://www.linuxquestions.org/questions/slackware-14/bootsplash-4175742241/