Ah that would be Zebra King, he's actually the first boss! There's 4 main bosses and additional hidden bosses, plus many more regular enemies to see!
And yes I will try and set up a downloadable version now! It will probably be an html file. Even though it will run in your browser, you won't need an internet connection to play it.
Just keep in mind that in order to save progress, you need to use the in-game save code system. Closing the tab will remove any progress.
I'll let you know when the downloadable version is up!