A few things have suggested to me that Piper may be herd bound... or Sienna bound to be more precise. She:
(a) jumped 5' fence into house yard where Sienna was grazing;
(b) calls when led out (onto road) and rushes to get back to Sienna;
(c) gallops and bucks along the fenceline when Sienna is led out;
(d) pulls free from tie, and gallops to find Sienna if she is out of sight/sound and today she broke my last clip on lead rope.
She is friendly, easy to catch, very willing (much more than Sienna has ever been) and will stand tied as long as Sienna is within sight/sound. If she thinks Sienna "is gone" she seems to panic, break free, call, gallop etc. Once she has re-joined Sienna she will generally just stand with her, kinda panting but fine to be led back and re-tied.
I'm concerned that if leading out is a bit of a problem now, then riding out in the future is going to be a BIG problem and what about when Sienna is no longer with us?
Do you think Piper is herd bound?
Any ideas on working through it?
Thanks for reading.