"Nature produces [Bernacae] against Nature in the most extraordinary way. They are like marsh geese but somewhat smaller. They are produced from fir timber tossed along the sea, and are at first like gum. Afterwards they hang down by their beaks as if they were a seaweed attached to the timber, and are surrounded by shells in order to grow more freely. Having thus in process of time been clothed with a strong coat of feathers, they either fall into the water or fly freely away into the air. They derived their food and growth from the sap of the wood or from the sea, by a secret and most wonderful process of alimentation. I have frequently seen, with my own eyes, more than a thousand of these small bodies of birds, hanging down on the sea-shore from one piece of timber, enclosed in their shells, and already formed. They do not breed and lay eggs like other birds, nor do they ever hatch any eggs, nor do they seem to build nests in any corner of the earth."
--Giraldus Cambrensis, Topographica Hiberniae, 1187.
I recall reading, back in my childhood, that some folks in Ireland had justified eating Barnacle Geese, (which are real geese,) during Lent on the grounds that they were fish (though perhaps I am confusing them with otters.) However, I definitely don't remember seeing them described as plants.
Another oddity worthy of Grandfather Herodotus is the fabled Vegetable Lamb of Tartary:
Although their bodies, noses, mouths, and eyes,
Of new-yeaned lambs have full the form and guise,
And should be very lambs, save that for foot
Within the ground they fix a living root
Which at their navel grows, and dies that day
That they have browzed the neighboring grass away.
Oh! Wondrous nature of God only good,
The beast hath root, the plant hath flesh and blood.
The nimble plant can turn it to and fro,
The nummed beast can neither stir nor goe,
The plant is leafless, branchless, void of fruit,
The beast is lustless, sexless, fireless, mute:
The plant with plants his hungry paunch doth feede,
Th’ admired beast is sowen a slender seed.
--Guillaume de Salluste Du Bartas, La Semaine, 1587.
I'd have thought it was just a fanciful description of cotton, but supposedly it's actually based on a fuzzy fern, the Cibotium barometz, part of which imaginatively looks like a lamb: