Chus are enemies from The Legend of Zelda: Twilight Princess. These translucent, amorphous, featureless jelly-like blobs are found commonly throughout Hyrule, especially in dark areas. Chus can merge with one another to progressively increase in size; when a merged Chu is struck, it will divide into two smaller Chus rather than perish. If Link fails to slay all of these fragment Chus quickly enough, they will rejoin into a larger Chu, forcing Link to repeat the process and start again. When a Chu is slain, a pool of Chu Jelly remains for a few seconds, which can be stored in a bottle and used for various purposes. Chus occasionally attack by dropping down from ceilings; when they do so, they emit their trademark squelching noise, allowing Link to dodge them if he is fast enough.
There are five varieties of Chus in all, distinguishable by their colors. Red Chus are fairly common, and their jelly refills eight hearts. Yellow Chus appear occasionally, and their jelly can be used to fill Link's Lantern. Blue Chus are rare, and yield jelly that restores all of Link's health. Purple Chus are encountered the most often; their jelly usually either recovers or drains a single heart at random, their jelly will rarely restore all hearts, and it may even even drain all of Link's health down to one quarter of a heart. Any time two differently colored Chus merge, they become a Purple Chu, greatly reducing the usefulness of their jelly. Finally, Rare Chus appear only a handful of times in the entire game. They are a slightly darker gold than Yellow Chus, and stand out due to their sparkling. Rare Chu Jelly has the same effect as Great Fairy's Tears: it replenishes all of Link's health, and increases his attack power until he takes damage or a certain amount of time passes.
In the Wii version of the game, Green Chus can also be encountered under certain circumstances. An artifact from when Twilight Princess still featured a Magic Meter, these Chus form when a Yellow and a Blue Chu merge. Upon a Green Chu's defeat, its jelly can be bottled like any other; however, it has no effect when imbibed, and its description is completely blank both when acquired and when examined in the Item Subscreen.
Large numbers of Chu can be found in the Lakebed Temple and under the large tree growing over the path near the Great Bridge of Hylia that leads to Faron Province. Yellow Chu can be found in Lantern Caverns, which is useful as their Jelly be used as Lantern Oil to refill Link's Lantern.
Interestingly enough in Twilight Princess HD, Green Chus can still be found, and Green Chu Jelly has an in-game description that does nothing but point out the fact that the jelly has no effect when consumed (presumably it was left in as an easter egg for players as it was given a description instead of being removed).
Ocarina of Time Beta
In pre-release footage from May 1997, enemies similar in appearance to the Chu appear. It is possible that the idea was reused for Twilight Princess. 
Twilight Princess Beta
Black Chus are another beta element that did not survive to the game's final release. While the enemy itself cannot be accessed, its jelly can be obtained via a cheating device such as an Action Replay. Its description notes that it "smells bad even with the lid closed", and consuming it results in the loss of one heart. The only surviving semblance to it is the Nasty Soup found in the cauldron over Coro's campfire (indicating that it may have been intended to be a Chu Jelly variant of it).