Arod is a "restive and fiery" (3.2.184) horse that Éomer gives to Legolas to ride. Poor Arod also has to carry the weight of Gimli, who is not at all comfortable with horses and clings to Legolas for safety. Legolas does not ride with stirrups or a bridle; as an elf, he can just speak to Arod and the horse will do his bidding. Arod bolts the night that Legolas, Gimli, and Aragorn camp at the border of the Forest of Fangorn. But he returns, along with Hasufel and Shadowfax, the next day.
Gwaihir is a huge eagle who rescued Gandalf from Saruman's tower at Orthanc in The Fellowship of the Ring. He reappears in The Two Towers as a spy for Gandalf. After Gandalf is reborn as Gandalf the White, he calls on Gwaihir to fly around southern Middle-earth and bring him news of the movements of the now-scattered Company. Gwaihir is the one who tells Gandalf of Merry and Pippin's capture and escape from the orcs. Legolas spots him twice, in The Fellowship of the Ring Book 2, Chapter 9 and The Two Towers Book 3, Chapter 2, but he doesn't know what he is looking at.
Gwaihir rescues naked Gandalf from the peak of Celebdil (a.k.a. Zirâkzigil), one of the Mountains of Moria, when he has completed his horrifying battle with the Balrog. Gandalf comments wryly, "Ever am I fated to be your burden, friend in need" (3.5.138). Gwaihir accepts this thanks good-naturedly, but really, it's totally true. Gwaihir saves Gandalf both from Saruman and from the Mountaintop. He is a handy transportation device, and he also brings Gandalf wherever he needs to be for the sake of plot advancement at key moments in The Lord of the Rings.
In our learning guide for The Hobbit, we said that the Lord of the Eagles was a deus ex machina—a farfetched plot device invented specifically to get the characters out of an impossible situation. We think that's true for Gwaihir the Windlord in The Lord of the Rings, too. He's always exactly where he needs to be at the right time to save Gandalf. He's a little too convenient, if we're being honest.
Hasufel is a dark grey horse that Éomer gives to Aragorn when they meet on the plains of Rohan. His previous master, Gárulf, was killed, leaving him free for Aragorn. But Hasufel disappears the night that Aragorn camps at the edge of the Forest of Fangorn. Legolas observes that the horses "spoke as horses will when they meet a friend that they have long missed" (3.5.8). Indeed, this proves to be the case because when Gandalf whistles for Shadowfax the next day, he comes running, accompanied by Hasufel and Arod. So Hasufel and Arod were just going to kick it with their friend and leader.
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Leaflock, along with Treebeard and Skinbark, is one of the only three remaining Ents from the Elder Days when the elves first woke them. Leaflock has grown tree-ish; he mostly doesn't even rouse himself during the winter any longer. He won't be much help in assaulting Isengard.
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When Sam first hears Gollum talking about what Sam calls the Swertings and the men of Gondor call the Southrons, he remembers a rhyme from his youth in the Shire:
Grey as a mouse,
Big as a house,
Nose like a snake,
I make the earth shake,
As I tramp through the grass;
Trees crack as I pass. (4.3.71)
Now that Sam is traveling so much, he still has hope that someday he will see this animal of legend, with its grey skin, long trunk, and enormous size.
And indeed, Sam's wish is granted in the very next chapter: as he watches Faramir's men battling it out with the Southrons, he sees a massive creature holding "a very large war-tower" (4.4.101) on his back. He trumpets and stampedes away from the battlefield, and Sam never finds out what happens to him.
This massive creature is like an elephant in our time, but "his kin that live still in latter days are but memories of his girth and majesty" (4.4.101). So, he's a lot bigger than your common elephant. The inclusion of the oliphaunt among the Southrons emphasizes that they are ethnically different from the men they are fighting. They come from far enough away that they have giant animals that remain only as legends in the parts of Middle-earth we get to see during The Lord of the Rings.
These are the rowan trees that grew in Quickbeam's home, "rowan trees that took root when [he] was an Enting" (3.4.138). They were enormous and produced much fruit to draw the birds. The orcs of Isengard came and cut them down. Quickbeam tried to wake them by singing their long names, but they could not answer. Poor Quickbeam.
Shadowfax is the horse of Rohan that Gandalf tamed in The Fellowship of the Ring, after his escape from Saruman at Orthanc. Éomer explains to Aragorn that Shadowfax is the most precious of King Théoden's horses, the best of the Mearas (Rohan's remarkable steeds, descended from the horse of Eorl the Young, which could talk). Shadowfax has returned riderless to Rohan seven days before Éomer's first meeting with Aragorn, but the horse is wild and will not accept a rider. Uh oh. Needless to say, King Théoden is extremely angry with Gandalf for making off with his best horse and then leaving him unridable by anyone except Gandalf.
Shadowfax comes to meet Gandalf when he whistles for him in the Forest of Fangorn. Together, they will ride into battle. When Shadowfax appears, Aragorn, Legolas, and Gimli understand the mystery of their disappearing horses: Hasufel and Arod bolted either from fear or love, but whichever it was, they soon found their way to their beloved chief, the head of the Mearas.
Skinbark, like Treebeard and Leaflock, is one of the only remaining first generation of Ents awakened by the elves in the Elder Days. His home is the closest to Isengard of the three, and Treebeard knows that he has been wounded by orcs. He has retreated to the mountains to be among his birches, and he won't come down again to join the fight against Isengard.
Quickbeam is an Ent who lives near to the meeting-place of the Entmoot; his tree seems to be the rowan tree. There are rowan trees growing in a circle around his ent-house, and he stops to sing to every rowan he passes. As a younger Ent, Quickbeam makes up his mind more quickly than all the other Ents about where he stands on the Isengard issue, so Treebeard brings him to Merry and Pippin as a companion. Treebeard adds: "Hm, hm, he is the nearest thing among us to a hasty Ent. You ought to get on together" (3.4.133).
On his walk with Merry and Pippin, Quickbeam explains that he is one of Skinbark's people. His forests have been destroyed by the orcs of Isengard. Merry and Pippin see this as justification for his quick decision to attack Isengard (as do we). He sings a dirge for his lost rowan trees, which sends Merry and Pippin into sleep.
When the Ents assault Isengard, it is Quickbeam who spots Saruman trying to escape from their wrath by hiding out in Orthanc. Of course, Quickbeam is particularly filled with hatred for Saruman because "his people suffered cruelly from orc-axes" (3.9.75). Quickbeam almost catches and strangles Saruman, but Saruman manages to get away.
The Entwife whom Treebeard seems to love the most. She has disappeared with the rest of the Entwives following the destruction of their gardens in the first war against Sauron.