Background ponies

Background ponies are ponies that appear in the background, and character models that are used frequently to fill out crowd scenes and incidental characters in the show.

Introduction
An episode is written, storyboarded, and has its dialogue recorded in pre-production. The storyboards detail each shot and most of the layout, posing, and camera actions. After storyboarding, backgrounds are created or re-used, the preliminary staging, composition, and color balancing is refined by the layout department, who ship the layouts to the animation department. After animation and animation revisions, the episode is further refined in post-production.

Background ponies
There is a differentiation between regular characters, supporting characters, and background characters.

Out of these three types, background characters are the most dynamic in their "deployment." With the exception of Derpy Hooves starting in season 2, these ponies are dropped into scenes only in the layout phase, after storyboarding has already finished. From a story standpoint, they are basically interchangeable and mainly serve the purpose of populating Equestria whenever ponies other than the main or supporting characters are needed. Compared to the more important characters, there is less of a focus on continuity and making sure every pony appears only once in a scene; in fact, crowd scenes often require so many ponies that they are filled with multiple copies of pre-existing designs (see below).

The design for all characters particularly their color schemes, are subject to approval by Hasbro. The background ponies mostly share two body types - one male and one female, and can thus only be differentiated from each other by mane style, mane color, coat color, eye color, cutie mark, horns and wings. The combination of all these factors has led to the creation of several hundred different ponies that have thus far been identified. Nonetheless, there are some "stock ponies" that are used in crowd scenes more frequently than others.

Examples of how background ponies are used
The diversity of the background ponies increases with time, likely because more and more variants were created during production. Even in later episodes, however, these background ponies are frequently "cloned," that is, their design (or an only slightly altered design) is used for multiple different filler ponies in the same shot. For example, in the picture to the left (from The Show Stoppers), one pony (Heartstrings) has been cloned five times to pad out the audience. In this case, the clones are actually slight variants of the default Heartstrings model, since they lack her horn. Straight clones from the unicorn variant can also occasionally be found in other scenes, however.

A more unusual example can be seen in the picture to the right: In The Ticket Master, a "mob" of ponies is chasing after Twilight Sparkle. The group consists primarily of 7 ponies (Diamond Mint, Berry Punch, Linky, Heartstrings, Lemon Hearts, Daisy, and Sparkler); in some shots, however, the entire group is "cloned" to make the number of her pursuers seem bigger. Three episodes later (presumably due to production constraints), the exact same "mob" was copied into a similar scene.

Background ponies are also frequently used as a way of providing brief visual gags that do not detract from the main action, but add to the experience of watching an episode repeatedly and looking out for these moments. For instance, the background pony Heartstrings is sometimes used by layout artists in unusual poses (cf. her gallery).