I posted this as a comment elsewhere, but it stands alone on its own.

Here's an explanation of all of the special non-standard variants in MWO. None of these are available for cbills, but, except for hero mechs, functionally identical mechs without their earnings boosts or special appearance are or will eventually available for cbills.

• Hero mechs don't have a parenthetical. Hero mechs have unique loadouts, unique paint schemes, and a 30% cbill boost. Hero mechs all have unique names (Oxide, Ilya Muromets, Medusa, etc.), and unique loadouts, including different hardpoints, different quirks, different engine/JJ caps, etc. Hero mechs are the only mechs that are different from regular mechs beyond for cosmetic differences and earnings boosts. There are a ton of these, and they can be bought with MC directly from the in-game store.

• (C) are champion mechs. These are (sort of) optimized pre-customized mechs with a 30% XP boost. (That applies to GXP as well.) These are built by forum posters and selected based on a poll. Except for their already-customized loadout, they're exactly like normal mechs; you can build the exact same loadout as a champion mech on a normal mech. These are sold for MC directly in the in-game store.

• (S) is special. All but one of these has a 30% cbill boost. The main way to get (S) mechs is through the Steam packages or more recent pre-order packages. The mechs from pre-order packages have a special paint scheme, but the Steam mechs just look like normal mechs. Except for the Steam packages, (S) mechs are always the first or Prime variant.

• The Jenner D(S) is Sarah's mech. Sarah was the daughter of a MWO player, but she passed away from childhood cancer. PGI held a fundraiser for the Canadian Cancer Society, and anyone who donated got a Jenner D with a special paint scheme and a 10% (later raised to 30%) XP boost (which also applies to GXP). This was a one-time deal; you can't get these any more.

• (L) is loyalty. All of these have a 30% cbill earnings boost and unique cosmetic differences. Every year in September, PGI holds a Customer Appreciation event and gives away special mechs to people who've spent real money in the last year. These mechs are new variants that are temporarily only available through the Loyalty program, although normal versions of the mechs (without the cbill boost and paint scheme) are available later for cbills. There's one for purchasing any amount of MC (Centurion AH in 2014, Cicada 3F in 2015), one for buying any preorder pack (Atlas S in 2014, Wolverine 7D or Nova D in 2015), and one for whaling out and buying max-tier packs or multiple large packs (King Crab 000 in 2014, Zeus 9S2 or Executioner C in 2015).

The older pre-order packs have special suffixes but they mostly work just like (S) mechs.

• (F) is founders, with a 30% cbill boost and unique paint job. (Originally, they were 25% cbills, but they got bumped up as part of the 2014 Customer Appreciation patch.) These mechs came from the original pre-order packages, way back before MWO even came out. Obviously, you can't get them any more. These are the Jenner D, Hunchback 4G, Catapult C1, and Atlas D. You can't get these any more.
• (P) is Phoenix, a pre-order package released around the same time MWO came out of Open Beta. These have a 30% cbill boost and unique paint job, but also have a 10% loyalty point boost in CW. (Also, you get an achievement for killing one of them in a non-Phoenix mech.) These are all mechs that were previously "unseen": they were based on designs licensed from anime series for the original version of Battletech. These are the Locust 1V, Shadow Hawk 2H, Thunderbolt 5S, Battlemaster 1G, Griffin 1N, and Wolverine 6R. You can't get these any more.
• (G) is gold. In the first Clan pre-order program, you could spend $500 USD to get a special gold-painted version of your mech. This program was, uh, not super well received at the time. Nevertheless, you occasionally see them running around, and they get a 30% cbill boost and they're, well, painted gold. Don't TK one, you'll totally get banned. They're otherwise the same as the (I) mechs from the first Clan pre-order package. You can't get these any more, and it's probably for the best.

The rest of these pre-order packages are still available for purchase, and only available on the website. Look there for details on variants. All of them have a 30% cbill boost and special appearance but are otherwise exactly the same as normal mechs.

• (I) variants are the special variants for the Clan, Clan II, and Clan III collections, as well as a special Mad Dog that was awarded as part of the 2014 Loyalty program to people who bought the top tier of the Clan and Clan II collections. Unlike the other Loyalty rewards, the special Mad Dog Prime is labeled (I) and is still available.
• (R) variants are the special variants for the Resistance and Resistance II collections, a pair of Inner Sphere packages released around the same time as the Clan collections.
• (O) variants are from the Origins IIC collection, a collection of "IIC" Clan upgrades of Inner Sphere mechs.
• All later collections just label the special variants (S).