Fillers
Fillers are horses that are used to fill a competition so another horse will be more or less guaranteed to win that competition. They are nothing special for breeding other than they can see the comps that the Blupping horse is entering. Often they are allowed to get quite low on health and morale to further ensure that they do not have a chance of beating the horse that they are filling for.
(c) info got from Loon Watcher on the forums
NIB
Non-Inbred - That means that no two horses can appear in the same family tree going back to Ouranos and Gaia.
IF
Immortal Foundie - Horses that have a Philosopher's Stone and born to Ouranos and Gaia.
Peggy or Peggi
Pegasus (horses with wings)
Uni
Uni Fail, No Horn, Non Uni, Uni Miss
Foals that have unicorn parents but didn't get a horn. these horses can be sold to the safe haven for 2,000e.
Well Covered (WC)
Covered Correctly. (only applies to unicorns)
Objective #
for an Objective
Preggo
755.69 F
It is very common for top breeders to name their horses in this format, which can include gender, skills, inborn skills, stars, amount of points for top skill, or genetic potential written in shorthand, e.g.
F 1998.02/450IBS/102* = Female with 1998.02 genetic potential, 450 inborn skills, 102 stars
M 800Dres/2000gp = Male with 800 dressage points and 2000 gp