There are a few candidates listed here which, bar the orc-only option, might fit with the clan/family vibe you’re looking for. Guild links are in the abovementioned thread as well.
There’s also The Family, for the less legal side to things. My impression is that they’re a fairly tightly knit group at that.
If you’re looking for something representing the multiracial militaristic side to the Horde, I’d also recommend looking into the Path of Glory. Sure there are rumours that they are bloodthirsty to a problematic degree, but I’m sure that’s just Alliance propaganda.
These are just my off the cuff suggestions and by no means an exhaustive list of options, and your best bet by far is to reach out and ask for details with the guilds themselves. What is and isn’t “casual RP” differs wildly depending on who you ask, for instance.