I'm surprised it's the Mule that's raising so much ruckus. They are such a determined lot, I hope she never crashes thru the gate.
She's teaching the gelding to be stressed over nothing
I wonder just who the Lead equine is - are you sure it isn't the "walk silent and carry a big stick" mule and she's having a first class hissy fit because you dare to take her BFF away?
My lead horse is fine when it's him that's going out of the gate. He's fine when I take either of the other TWH's out of the gate.
If I take his Arab BFF of 20+ years out of the gate, without him, oh my goodness, you'd think Armageddon was coming.
It's gotten to where the lead horse has won because he is coming 26, has hind gut ulcers and colicked nine times in 2012. I've had him 22+ years and I'm sure the snot face knows I am scared to death he will colic if he gets too stressed.
Since you're dealing a mule, and mules don't stray mentally, I don't have much of an answer, accept to try and take this horse out daily if you can and make her "disappear" for longer periods of time, each day