The Pronghorn antelope (Nature 1991;353:748-750) is capable of reaching the dazzling speed of about 100 km/h, second only to the cheetah, and is even more interestingly capable of running 11 km in 10 minutes, at an average speed of 65 km/h. 
Its maximum oxygen uptake (VO2max) reaches up to 300 ml of O2/kg/min, a value that is about 5 times higher than the average for similar sized mammals (32 kg body weight). 

Such excellent aerobic performances are due to the antelope's elevated cardio-respiratory efficiency, a high Hb concentration and an elevated mitochondrial mass. 
But there is more. 

An interesting study by A.S. Popel et Al. (J. Appl. Physiol. 1994;77:1790-1794) compared "athletic" mammals (horse, dog) with "sedentary" mammals (cow, sheep), of similar body mass, further matching them with the Pronghorn antelope. 

This study detected that red blood cells of "athletic" mammals have an elevated aggregation capacity, whereas comparable "sedentary" mammals lack this characteristic almost entirely. 
Pronghorn antelopes showed an even higher capacity of red blood cells aggregation than dogs and horses, which, while being "athletic", have a definitely lower aerobic capacity (VO2max = 136 ml of O2/kg/min). 

Red blood cells aggregation corresponds with the tendency of such cells to line up in "rouleaux", one after the other like small train wagons, whenever there is a low shear rate as in venous post-capillary microcirculation. 
This particular disposition distances RBC from the venule walls, reducing friction and facilitating the venous return to the heart, further limiting the accumulation of fluids in muscle tissues. 

Elite human athletes (Clin Hemorheol Microcirc 2003;28:139-149) were also measured after an effort with a high capacity of RBC aggregation, correlating significantly with the capacity of performing aerobic work.  
In humans, such aggregation seems to be related to levels of plasmatic protein, especially FIBRINOGEN and ALBUMIN (Am J Physiol Heart Circ Physiol 2003;285:H2663-2669). 

Elevated levels of TRIGLYCERIDES have also shown to have an effect of increasing RBC aggregation, as in the case of a meal rich in lipids (Clin Hemorheol Microcirc 2004;30:301-305). 

Such interesting observations may therefore help explaining some of the amazing improvements in aerobic performances (VO2max: +11.1%; Running Time to Exhaustion: +20.3%) noticed in athletes after a FAT LOADING procedure (Med Sci Sports Exerc 1994;26:81-88).