Firstly, it's a great capture. But it's not one I would ever consider basing an opinion on there being something possibly wrong with your lens.

Here's a few of my observations/ideas regarding it's possibly perceived lack of IQ by some;

a) According to the Exif it was shot at 340mm and f/5.6. I'm not familiar with this lens but in general getting near max FL and wide open is not going to be the sharpest, but can often be compensated in post.

b) There appears a fair amount of noise in the image although shot at ISO400 that may have affected sharpness. Looking in the Exif at the in post work it shows the following,

Exposure 2012 +0.60
Contrast 2012 0
Highlights 2012 -100
Shadows 2012 +100
Whites 2012 +38
Blacks 2012 0
Clarity 2012 +21
Dehaze +24

If this is correct, I suspect increasing exposure, lifting the shadows 100% along with the added clarity and dehaze has not helped.

c) What I do find interesting is that the horses all look sharper than any of the ground/background foliage around the same focus plane which makes me wonder if you were very slightly panning this shot to achieve best frame positioning for the group.

d) The harsh lighting also didn't help.

Interesting to hear others thoughts on the above.

Look at the image details at 100%. The lead horse looks great, from say the lead leg / shoulder through the eye of the nearest horse immediately behind. Imagine the image if all the horses had this same level of detail. The EXIF details show possibilities. As noted by others, 1/200 seems a bit slow for this moving group. Based on the horses relative to the static vegetation, the panning technique seems excellent as well as the IS support for the lens. Probably a bit faster shutter is an option to better freeze the horses. The EOS data has been striped in the processing, so we can't see where the AF point(s) were located nor the AF mode. But still, where the focus is sharpest, the image is great for this distance and the fine details where the focus is sharpest.

But more important than a faster shutter would be a smaller aperture, both for the sharpness of the lens and for a depth of field to cover this group. Look the final trailing horses in the left of the image. They're not soft from being too slow on the shutter. Rather, these horses are outside the depth of field at f/5.6 for a lens that isn't sharpest at f/5.6. Something in the f/8 to f/10 range would be a better choice for sharper results at 300mm and longer, and to extend the depth of field, even keeping the same shutterspeed at 1/200.

Let's see - horses galloping at a fast pace, obviously a good distance away (quite far for a 400), panning evident from foilage blur, nobody is posing....looks pretty damn good to me! Grahame noted some exif details for thought. Perhaps a increase in shutter speed - in that harsh light faster (maybe 1/500 to 1/1000) would freeze the horses more and you'd still have some panning/movement blur), increase exposure somewhat and an ISO dip.... At that distance there may be some atmospheric distortion too (like heatwaves on a highway), and Chg Canon mentioned DOF, another consideration.

All that aside, from a reasonable viewing distance (like on a wall) it would convey the scene well, I'd hang it. Sharpness while relevant, is not always the greatest concern.... AND it is easy to pick at things when "recliner quarterbacking" a scene. Put it on a larger monitor or TV screen and stand back and look at it... from 4 feet at 22" wide on my monitor it looks great - if it makes you happy and conveys what you desired, then it's a keeper!

My wife noticed a couple of the mares look pregnant (maybe)... hope they foal well...and all get to run like the wind!! Thanks for sharing.

I recently posted some pictures of wild horses taken on the Yakama Indian Reservation. The horses are extremely wild. I just learned it's because they've been shot at. Consequently, the pictures are taken in RAW at 400mm with my Canon 400 L lens. I love the lens but one of the Hogs responded that they were soft and suggested there may be something wrong with my lens or maybe my technique. I'd really like your opinion. I know they're not tact sharp. Is it because I'm doing something wrong or am I just pushing my lens to the limit? I'm posting a picture as taken as well as the cropped version to help you know how far out the horses are. My husband, a hunter says he thinks they were about 1/2 mile away.
Thanks for your time and suggestions.