You can look at the source code of the ones included with Haiku to see what a screensaver needs in order to work on Haiku:
http://dev.haiku-os.org/browser/haiku/trunk/src/add-ons/screen_savers

And/or check out the sample ones available on BeBits: 
http://bebits.com/app/3019. 
Under sample-code/screen_savers, there is 3 screensavers. "Spots" is a very simple saver, which draws black holes over the desktop. Here is your starting point.

From here, you'll need to know better how to draw something on a BView:
http://www.haiku-os.org/legacy-docs/bebook/TheInterfaceKit_Drawing.html

From there it's up to you to add your own flare.  Show us what you can do.

 If you are new to Haiku. you'll need to install Haiku either on real hardware or in a VM, with enough hard drive space to do some work, perhaps around 2-3GB should be fine.  The recently released R1Alpha4.1 should work just fine for this task.