If you've ever been snorkling or scuba diving in the Caribbean, you will have noticed waving fields of things that look for all the world like plants. They grow out of the sandy or coral bottom, they are branched, and they sway gracefully in the current.
Plants they are not, though. You are looking at animals called gorgonians, also known as sea fans or sea whips. They're related to coral. (Sorry for citing to Wikipedia. Wikipedia is a good reference!)
What makes them animals and not plants? Among other things, they can't make their own food. They have to catch plankton and eat that. (Actually, lots of gorgonians do have symbiotic relationships with photosynthetic algae that produce some food for them, but in no sense are gorgonians plants.)
When Mark Catesby visited the Bahamas, he had no way of knowing that the sea fans he saw were not plants. They sure look plant-like! So he collected a gorgonian, and sent it off to London with his other herbarium specimens. That is why there are animals in his herbarium at Oxford.
