Forest law is a curious artifact in British legal history. It's a long-since defunct legal system intended to allow the aristocracy to get drunk in peace, which inadvertently became one of the first organised attempts at ecological conservation in Britain. Incidentally, it's also part of the reason why only the Queen is allowed to eat swans.
...perhaps I should start at the beginning. In this case, the beginning is the Norman conquest of England. See, the Normans were big on hunting, much more so than their Anglo-Saxon predecessors. William the Conqueror, the first Norman king of England, was no exception, and he took exception to people interfering with his hunting, to the extent that he created an entire system of law to let him get on with it. Forest law was based on royal prerogative, the ipse-dixit 'because I say so' of British law. As such, it stood apart; you had common law, canon law for the Church, and forest law. Essentially, the idea was that the Crown could designate an area of land as a royal forest; it didn't actually have to be a forest, but once it was designated as such, messing with it in any way, shape or form made you the King's bitch. So to speak.
There are two main categories of offences in forest law, offences against the vert, and offences against the venison. In other words, interfering with the plants, or the animals being hunted, most usually deer and hares, sometimes boars or wolves. This is all very practical if you want to keep an area for hunting; it forbids anyone from poaching your game, or screwing with the plants they eat to survive. Ideally, you thereby keep your game from being hunted to extinction. Not that this worked perfectly, which is why there aren't any wild boar or wolves left in Britain these days.
Of course, the problem with forest law is that it was something of a power grab. For one, it cuts off a (usually pretty substantial) area to peasants for whom poaching was a vital source of food and income. As such, it wasn't popular, but no-one wanted to get William too exercised about it, given his temper. For another, once it was in place, the law kept broadening, as the aristocracy started finding new things to hunt, like foxes, and new things to ban, like cutting down trees. In addition, extending the right to declare an area a royal forest became something of a reward the crown could use for nobility. Between the King and local lords, at its height, forest law was applied to around a third of the south of England's land area.
Needless to say, this rankled people, and their bitching about it shows up in The Rime of King William, a verse in the Anglo-Saxon Chronicle that serves as an obituary of sorts:
He was fallen into covetousness, and greediness he loved withal. He made many
deer-parks; and he established laws therewith; so that whosoever
slew a hart, or a hind, should be deprived of his eyesight. As
he forbade men to kill the harts, so also the boars; and he loved
the tall deer as if he were their father. Likewise he decreed by
the hares, that they should go free. His rich men bemoaned it,
and the poor men shuddered at it. But he was so stern, that he
recked not the hatred of them all; for they must follow withal
the king's will, if they would live, or have land, or
possessions, or even his peace. Alas! that any man should
presume so to puff himself up, and boast o'er all men. May the
Almighty God show mercy to his soul, and grant him forgiveness of
I wasn't joking about the 'King's bitch' thing earlier. Kill a deer? You're getting blinded. It wasn't informally draconian, either; forest law was a fully developed system of law, with separate courts, magistrates, an organised system of gamekeepers that you could make the case was one of the first legally-enshrined police forces in British history, and a whole bureaucracy dedicated to preserving the forests for hunting. Forest law had a distinctly commercial aspect, too; the Crown could - and did - licence the right to fell trees, or build pastures, or extract any kind of natural resources from a royal forest, which could be highly lucrative business. In effect, this was a monopoly on huge swathes of southern England. In fact, it was sufficiently reviled that a fairly decent part of the Magna Carta is a restriction on just how far and how unfairly the King can enforce it.
By the 14th century, forest law was pretty much out of fashion, but like all kinds of anachronisms, it got dragged out every now and then when convenient. It's now long since gone, forestry in Britain having been managed by government agencies and royal commissions for the last couple of centuries. Still, its most enduring consequences have been beneficial, and utterly accidental. By legally walling off huge areas from the general public, forest law prevented development from encroaching upon them - a number of royal forests have survived to the present pretty much intact, and are now in the commons. The New Forest and the Forest of Dean were both royal forests, and the system of law underpinning their survival also set the ball rolling on public parks - Richmond Park in London still has deer in it to this day. Which brings me back, finally, to the swans. The Crown lays claim to ownership of all mute swans on the river Thames. This is mostly a formality; one of these strange things that hang on in English law years past their expiration date, like the death penalty for arson in the Royal Dockyards. Still, if you ever find yourself by the Thames of an evening, feeling a bit peckish... remember the whole getting-your-eyes-put-out thing, and leave the swans alone.