Freshwater Plimpies are magical creatures that can be found in the stream near the Lovegoods' house (DH20). They are either a special Plimpy breed or an alternative name for Plimpies. The first is more likely, because ordinary Plimpies are known to inhabit deep lakes, not streams (FBAZ). Xenophilius Lovegood has a special recipe for Freshwater Plimpy soup, and claims that it is always requested by others (DH21).
When Harry Potter, Ron Weasley and Hermione Granger visit Xenophilius, he claims that his daughter Luna is fishing Freshwater Plimpies at Bottom Bridge. Luna has really been taken hostage by Death Eaters. The Trio discovers his falsehood when he brings only four bowls of Freshwater Plimpy soup (DH21).