Ragwort

Cinnabar Moth caterpillars on Ragwort

These clusters of sunlight
  shine with the wings of bees
  but are dark honey when
  hidden in sweet hay.

  I’ve seen them, untouched,
  seeds drifting over close-bitten grass
  where horses graze. Caterpillars
  of cinnabar moths
                          dependently displaying

  In their tiger stripes its dark
  And its sunny aspect, the black
  And red moths returning to lay
  Eggs where no ruminant rasps them.

Ragwort is poisonous to cattle and horses who have the sense to avoid it when growing in fields where they graze, but if too much of it gets into hay it can cause problems.
The Scottish poets Robert Burns and James Hogg both have poems where the stalks of Ragwort are ridden by fairies or changed into horses by witches.
The specific name 'jacobaea' apparently comes from the fact that the plant is in full bloom on St James' Day (25 July).