The deep cotyloid cavity renders disarticulation difficult and luxation does not often take place.
"Lameness of the Horse" by John Victor Lacroix
The cotyloid ligament being broader and thicker than usual, makes the osseous portion of the socket appear deeper than it really is.
"Manual of Surgery Volume Second: Extremities--Head--Neck. Sixth Edition." by Alexander Miles
Another peculiarity of the mammalian pelvis appears to be the cotyloid bone already referred to.
"The Cambridge Natural History, Vol X., Mammalia" by Frank Evers Beddard