I’m not sure how familiar people are, outside England, with straw art (corn here means grain, just to avoid confusion amongst Americans).
Corn dolls or corn dollies might be a little more familiar, these are woven, folded and knotted straw shapes (or literal dolls) with a hollow centre, made – traditionally – from the last sheaf to be harvested. The spirit of the field, the spirit of the corn, is supposed to have been made homeless by the harvest, and the corn doll provides a home for the spirit until sowing season returns. When it is time to sow again, the old corn dolly (which may have wintered hanging from a cottage beam) is ploughed under so the spirit can inhabit the new field.
Corn stalk sculpture with a bit more flare, thatch finials are demonstrations of the thatcher’s art. These complex figures are advertisements for the thatcher’s craft and traditionally were made from a combination of stalks, ash and hazel. In the modern era they’re woven into and through chicken-wire (which also tends to be used to shape and protect thatched roofs as a whole).
Not only was this artistry eye catching, but sometimes they were made into straw weathervanes, rotating on wooden or metal spindles to show the direction of the wind. One claim is that the shapes of animals also act as scarecrows, stopping birds – and other animals – pilfering the straw from the roof for their nests or searching for missed grains. I must say, in my experience, most birds that perch on these figures don’t give a damn, and jackdaws seem to take special delight in perching on the ears of straw rabbits or the noses of straw foxes.
It’s probably just more marketing on the part of canny thatchers, but it is also claimed that having a thatch finial is protection against witchcraft, much as having a horseshoe over the entrance (open at the top) to catch luck.
In a fantasy setting corn dollies could provide a bonus (advantage in 5e D&D terms) to the farmer’s when they plant and reap their crops. The loss of a corn dolly to theft, arson or for other nefarious reasons, could provide an excellent plot hook for a more medieval or low magic setting. A corn spirit – bound into the dolly – could also be a somewhat powerful magical ingredient and a hard thing to convince villagers to part with.
The thatch finials, if you believe the marketing of the thatchers, could provide protection against magic while you are under that roof. Whatever the system you could grant a +1 bonus (5%) to saving throws, evasion, resistance or whatever your game system of choice happens to use.
A little bit of colour and a little bit of magic up on the roof.