Purple Podcast Icon

OK: and what is the icon supposed to represent - presumably an application? - only Apple's Podcasts application isn't due until Catalina is released in the autumn: AFAIK in Mojave you use iTunes as with earlier systems, unless you've installed a third-part application.

You can place any application in the Dock (not the menu bar) simply by drag its icon to the Dock (this wont affect the actual application). Apps only appear in the menu bar (on the right-hand side) if they are specifically designed to do so.

What operating system are you using?