Any ideas for this mane?

A running braid is easy, looks polished, and is probably your best bet!

It's bascially done the sameway a french braid would be done on a human (or your horse's tail), and I find it's most easily done (and looks the best) while sitting the horse's back! This way, you're braiding towards yourself so the braid goes directly along the horse's neckline. I find it stays in better using this method as well.

Start off by hopping on your horse (or on a mounting block, whatever suits your fancy) and brushing the mane towards you (if you're mounted, it will take a minute and look funky but it'll work out :P). I'd suggest soaking the mane with a little water, Quick Braid, or even watered-down human hair gel, to make it tacky and prevent fly-away hairs . Start off with a regular 3-strand braid at the top of the mane, keeping it very tight. After all 3 strands are incorparted in a few times, grab another small section of mane and add it to the section you're about to braid in (I'm terrible at explaining, sorry >__<) Repeat this, keeping it very tight, until you reach the end of the mane. Once you run out of mane to add, keep braiding until you reach the very end and then secure with a rubber band. Spray with a little hairspray to keep it in place and you're all set!

Be sure to practice beforehand, once you get the hang of it it'll only take you about 15 minutes from start to finish. Good luck!

Arab plait?

https://www.google.co.uk/search?q=arab+plait+horse...

Or if your planning on doing a lot of showing I would pull the mane so you can have show plaits.