MG Horde is active, and there are various active RP guild options. We tend to be as a community story-focused, and less about random walkup rp. Any number of members of the active Horde RP community can attest to things being fairly active. We are generally a laid back adult community.
The Horde Faction RP community recently launched a project, The Horde Vanguard. You cans see more about it here: ⫸ The Horde Vanguard - Horde RP Community ⫷
Here is a screenshot from the Horde deployment to Conquest, a current D20 rp campaign being run on MG. This screenshot is from last week.
If random walkup rp is your thing, the single best place to go in all of WoW is Alliance on MG.