Quote from : Harry Potter and the Deathly Hallows Book

And then a silver hare, a boar, and a fox soared past Harry, Ron, and Hermione's heads: The dementors fell back before the creatures' approach. Three more people had arrived out of the darkness to stand beside them, their wands outstretched, continuing to cast their Patronuses: Luna, Ernie, and Seamus. "That's right," said Luna encouragingly, as if they were back in the Room of Requirement and this was simply spell practice for the D.A. "That's right, Harry... come on, think of something happy..." "Something happy?" he said, his voice cracked. "We're all still here," she whispered, "we're still fighting. Come on, now..." There was a silver spark, then a wavering light, and then, with the greatest effort it had ever cost him, the stag burst from the end of Harry's wand.