Thanks, everyone, esp. Peace. Here are the lyrics:

The Merry Nightingale

The nightingale, the merry nightingale,
she sweetly sits and sings and sings.
The pretty nimble doe doth trip it to and fro.
The stond horse kicks and flings and flings.
The cuckoo he doth fly from tree to tree,
and merrily through the wood,
"cuckoo! cuckoo!" rings.

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Click to play

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I believe that the horse is tethered, and it being spring, the horse is feeling frisky and kicks, flinging its legs out.

Uncle DaveO, you were right. Stallion comes from the OF estalion and the OGH stal, a stable. Who'd a thunk it?