I forget, is that kind of swishing an indicator of estrus?

BlastingNaba said in comment #2573878:

I forget, is that kind of swishing an indicator of estrus?

Tail is held up higher (and kept there) for that and the swishing would be slower (see post #5457543). Not so much swishing as "presenting" their... *ahem* bits.

The Grooves are doing the classic fly-swatting swish. Which when isn't done with flies around is an indication of annoyance or mild discomfort.

For Dura... that kind of tail "wringing" usually indicates a horse in extreme pain. Some kind of chronic injury like a bad back, or badly-fitted equipment like a mouth bit or saddle. Though some weirdo horses do it inexplicably anyway much to the consternation of their worried owners who spent mountains of vet bills just to rule out every possible cause of pain.

And Dura IRL was indeed a "weird" horse. (Don't think he does the tail wringing though.)