What defines a Crabbet horse?

In technical terms, a Crabbet Arabian is a horse that traces in all lines of its pedigree to
Arabians owned or bred by the Crabbet Park Stud, whether under the management of
Lady and Lord Blunt, Lady Wentworth, or Cecil Covey.

Crabbet Arabian bloodlines show off their virtues especially well in saddle competition and in endurance riding.
The Tevis Cup in the United States and the Quilty Hundred Mile Endurance Ride in Australia are dominated by Crabbet Arabians.

They excel at the highest levels of performance competition due to their strong bones, excellent conformation, graceful movement, good temperament and outstanding performance ability, which are the recognized virtues of Crabbet Arabian horses.

Approximately 90% of Arabian horses alive today have bloodlines that trace back to Crabbet horses, particularly Mesaoud and Skowronek. Crabbet Park Arabians were exported all over the world. Even the Egyptian Agricultural Organization has a large percentage of Crabbet horses originally purchased from the Sheykh Obeyd stud.

You can read more about Crabbet horses at those pages:

Crabbet - the Early Days

"Crabbet Arabian Stud" acc. to Wikipedia

How to count the Crabbet % for your horse

Crabbet Convention 2002

Pilgrimage to Crabbet Park

Crabbet Stud Tour Part1

Crabbet Stud Tour Part2

Non Crabbet arabians within the UK part1

Non Crabbet arabians within the UK part2