For those of you keeping track, the phrase "red fern" doesn't appear until the seventh paragraph of the last chapter. And it only comes up 12 times. Twelve times in over 200 pages! Hm. It's almost as though what Billy learned from the dogs is more important than the dogs themselves.
Let's start with looking at what, exactly, the red fern is.
According to the book, there's a legend that only an angel can plant a red fern. The legend is a bit unpleasant, since it's about two Native American children who got lost and froze to death. (See, we said it was unpleasant.)
When the kids are found in the spring, a red fern is growing between their bodies. This makes the red fern sacred. In this one little legend, we've got a connection to the spiritual and sacred, an allusion to religion, and an implication that the dogs really were sent by God (see themes for more about Religion). That's quite a wallop to pack into one little legend.
Okay, so let's say we're not into all that kooky supernatural stuff. If we're just talking about a literal place where a red fern is growing, then we're still talking where Old Dan and Little Ann are buried—the place of Billy's childhood dreams and ambitions; his first true love; the proof of his character; and the ultimate sacrifice the little dogs made so that he and his family could better themselves.
Hey, where'd those tissues go?