RP, in my head I actually have the big idea I think you're getting at sorted into a few different categories. The cowherd (regardless of gender) is someone who is vying hard to tear up a poly relationship, consciously or not, and make off with one partner into a mono setting. Then there's people who have trouble acknowledging that their actions impact others, with cowherds perhaps being a subset of that larger group. Then there's the people you wish you weren't in a constellation with, but you're not the boss...
I guess people actually "tolerate" people in a poly constellation. I guess sometimes there is no choice but to "tolerate" if a person comes in who is invited by someone other than me and I am not the one making the choices to have them there. How does one deal with that?
Our constellation has a dozen people in it (more if you count infrequent partners), and I have one metamour I've been bumping up against. He and my girlfriend have been emotionally compromising each other for years (which could be read as: he's been breaking her heart, and she's been kicking his around, and I've been stuck picking up the pieces of her afterwards, usually as her friend rather than as her gf). Now they're in a position where they can date, because all their other partners are poly, for the first time ever, and I'm waiting for the other shoe to drop.
I spent the first month or two doing my best to be happy about it for her sake, after I frankly told him why I was concerned the weekend they started dating. Then I realized that:
A) I'm certain he doesn't respect my relationship with her, which is admittedly semi-understandable because it's complicated. (We've taken to calling ourselves 'girlfriends from the waist up', and like to debate about whether either of us is actually bisexual.
B) I've seen evidence that leads me to believe he doesn't respect her, primarily from how immaturely he's handled their disagreements. (I'm talking blowing off phone dates in a long distance situation, severely passive aggressive text messages, not talking for a week and then apologizing only for the text and not the avoidance, and not wanting to talk about what to do next time they disagree, all while she's finishing her last semester of college and under enough stress... Red flags are waving in the breeze.)
If I were dating him, he'd be history. Thing is, I'm not dating him. She is, and I care about her deeply and usually respect her opinions. I'm honestly not sure what to do about him; he and his boyfriend are the only long-distance people who are significantly tangled into our constellation, so I almost never see him, and when I interact with him it's almost exclusively through her on the telephone. Without time to actually go face to face, I can't figure out how to start caring about this bastard. I can't even tell him I think he's behaving poorly, because we don't have a way to talk other than facebook.
The best I've been able to do is tell her I can either play fair and unbiased and keep my opinions out when he comes up in conversation, or I can tell her exactly where I wish she'd shove it on him. She gets to pick which one, and then I either play the role politely or I mouth off-- and either way I do my best to calmly tell her what I would and wouldnt' stand for if I were in her shoes, and that she can do whatever she likes with that knowledge.
As for cowherds... No clue. Haven't met one, would kick 'em to the curb if I did and had the luxury. (Again, metamours make everything complicated.)