"Yea, it's the intro for a story I'm working on. Basically, I have a group of seven humans piss drunk in an Equestrian bar, and I'm kinda having trouble with how to end it. Eventually, the guard will come along and escort them home, as this has become a regular thing with them on Sunday nights."

"What I need, is some random OC or bar pony to have some kind of problem with them; something to complain about. I would normally be able to handle this fine, but me brain is deciding now it a good time to get diarrhea and not being of any help. So, suggestions? Please?"

"You get imaginary cookies if you help!"

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OC pony: My god, they're too noisy, they are so buckin' rowdy that they break almost all of the glasses onto the floor. Someone, please kick them out.

Is that what you're looking for?

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Hmm... more like trying to pick a fight. Also, I kinda need the character to come back in a later chapter; so slight bit of insight of looks and character would be helpful.

They should make fun of Derpy and The Doctor and her kids get really upset

A bunch of Humans walk into a Pony bar and sit at the counter.
The first orders a beer. The second orders a shot of whiskey. The third orders an apple martini. The fourth says he's the designated driver, and the fifth asks for a mug of hard Cider.
The first Human receives his beer The second is told they don't have whiskey in stock right now. The fourth is given a complimentary water, and the fifth receives his cider.

The third receives a very confused Apple Bloom in the largest mug the building has.
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Silliness aside, for the issue you're having, how about a pony with a name that doesn't fit his occupation in the slightest and is constantly belittled for it.
If that's not serious enough for what you're going for, How about a scientist whose work is looked down on. He/she is trying to build a rocket to get to their own moon (after one of the Human's previous visits and declaration that their species had flown to and walked on the moon, and then flown back to Earth again) but his/her peers argue that since they can already get there via magic there's no need. Because of this he/she gets very little funding.
Or you could go for the classic/cliche story of the person whose spouse is leaving them for someone else, but in this case you can make it to be for a different species.

Oh wait, just understood what you wrote...Well, it could be a case of one of them having slept with somepony's sister. Several times. A sort of 'defending her honor' thing.

have them go off on how bad ponys smell have you ever been to a horse stable?lol it not the sweetest smell in the world or just have them make fun of ponys for not having hands and having to use there mouths or for being pussy herbivores