Seems ok for horses - only issue with very large quantities verified by other websites, but the amount a horse would ingest while grazing shouldnt be an issue, and is nutritional in moderate quantities.
I’ve looked at some photos of chickweed on google and it doesn’t really look like that I don’t think. But maybe it’s the stage of growth it’s in. I’m not particularly worried about it being toxic more about it not being palatable and taking over from grasses thereby reducing our grazing which has taken a beating over the last few years with very little summer growth and wet winters poaching the ground.
ETA it does look like the sticky mouse ear variety though now I’ve specifically googled that.