In the Howrse world there are a great many abbreviations that are used by long-time members.  There is definitely a learning curve involved to know what people are talking about.  I've listed the most common ones below but feel free to add any I have missed.  (Only abbreviations related to Howrse will be published.)

• Admin - Administration, Employers of Howrse.
• AL - Apollo's Lyre, a black market item that allows an owner to customize a horse's name. 
• Appy - Appaloosa.
• AT - Aphrodite's Tears, a black market item that raises the IBS of a foal by 50%.
• BC - Bold Characters, when a skill's potential has been achieved the skill is considered bold.
• BLUP - Best Linear Unbiassed Potential, BLUP is complicated.  Basically, the higher the BLUP (on a scale of -100 to +100) the better the GP of the foal.  A horse's BLUP rises with training, competitions won and age.
• BM - Black Market Item, special items in the black market that can only be bought with passes, earned in contests or traded for in the item exchange.
• BP - Black Pearl, a special item in the Black Market that is offered occasionally that allows players to create and submit their own designs for Golden Apple coats.
• BUMP - Bring Up My Post, a way of getting your topic back up to the top in the forums in case nobody is responding (use with caution, too much use and you can be sanctioned).
• CT - Chronos Timer, a black market item that allows an owner to train their horse twice as fast.
• EC - Equestrian Center, after a player reaches 20 days' seniority they can create and run their own equestrian center where they can board horses, plant crops, etc.
• FW - Fertility Wand, a black market item that causes a mare to have twins.
• GA - Golden Apple, a black market item that allows an owner to place a special coat on their horse.  The Golden Apple also extends bonuses for competitions.
• GMT - Greenwich Mean Time, the time that the game is in.
• GP - Genetic Potential, a number assigned to all horses when they are born based on the Genetic Potential, Skills and BLUP of the parents.
• GV - Gypsy Vanner.
• HoP or HOP - Horn of Plenty, a black market item that gives you random items for your horse.
• IBS - Inborn Skills, skills a foal has when they are born.  The higher the GP, skills and BLUP of the parents, the higher the Inborn Skills.
• IS - Innate Skills, also known as Inborn Skills.
• MA - Morpheus Arms, a black market item that allows the owner to age their horse indefinitely without the use of aging points.
• MB - Medusa's Blood, a black market item that turns a regular horse into a pegasus.
• Mod(s) - Moderators, players with high seniority that have a great deal of game knowledge and are active players are sometimes asked by Howrse Administration to be moderators in forums.  They watch the forums and other areas to make sure all players are following the rules.
• Newf - Newfoundland Pony.
• NIB - Non-Inbred, a horse that can trace it's lineage back to Ouranos and Gaia without any duplications in Mare or Stallion breeding.
• OT - Official Topic, a topic posted by an Admin or Mod that is found at the top of the forum.
• PB - Purebred, an animal that is 100% one breed.
• Peggy or Peg - Pegasus, a horse with wings, obtained by placing Medusa's Blood on a horse.
• PM - Private Message, a message between two players sent through private mail.
• PoC or POC - Piece of Cloud, a black market item that brings your horse back 4 months and credits you 2 aging points.
• Prego - Pregnant, a mare carrying a foal.
• PSH - Purebred Spanish Horse.
• PStone or PS - Philosopher's Stone, a BM item that makes a horse eternal when placed on the horse.
• QH - Quarter Horse.
• QP - Quarter Pony.
• RC - Retired Coat, a Golden Apple coat that is no longer available to purchase with a Golden Apple.  Is shown by a silver apple on the horse's page.
• RI - Riding Instructor, one of the jobs available in the equestrian centers.
• SDB or SB - Standardbred.
• Stally - Stallion, male horse that can breed.
• TB - Thoroughbred.
• Uni - Unicorn, a horse with a horn, the horn gives the horse a dressage bonus.
• WoY - Water of Youth, a black market item that causes a horse to age by one month every time they age instead of two months.
• ZBolt - Zeus' Lightning Bolt, a black market item that changes the gender of the horse.

There are probably many other abbreviations that I have not covered.  Feel free to add to this list.  Hope this helps.