I have noticed that some fat has a more waxy texture. I have found this in the body cavity of lamb, on beef ribs, and especially on beef heart. In a bucket of beef tallow (that was liquid and allowed to solidify in the bucket) I have found that the fat near the top is waxier than the fat at the bottom, so I suspect this waxy fat may have sorted itself out by different density or freezing point.
What is the chemical property that makes it waxy? What is the biological reason for (some kinds of?) animals to accumulate it in some areas of the body? Is it normal to have trouble digesting it? Are there any other health implications to eating it?
What is the chemical property that makes it waxy?
Stearic acid. It's very waxy. Candle wax and crayons are made from it.
In cattle, waxy fat contains more stearic acid and less oleic acid. Oleic acid is unsaturated and liquid at room temperature, so the less there is, the harder the fat.
Here's a table that compares fat from three places in the bodies of cattle. As you can see, the waxier fats have about twice as much stearic acid (C18:0) and 25% less oleic acid (C18:1).
Here's a quote from a study about lamb fat:
...perinephric fat [suet] contained more stearic and less oleic, palmitoleic, palmitic and myristic acid than subcutaneous fat.
Here's a table from a different study about lamb. The numbers circled in red show that kidney fat (suet) contains more than twice as much stearic acid as some subcutaneous fat.
Ashes JR, Thompson RH, et al. A comparison of fatty acid profiles and carcass characteristics of feedlot steers fed canola seed and sunflower seed meal supplements protected from metabolism in the rumen. Aust J Agric Res (1993). 44; 1103-12.
L'Estrange JL and Mulvihill TA. A survey of fat characteristics of lamb with particular reference to the soft fat condition in intensively fed lambs. The Journal of Agricultural Science (1975), 84:281-290.
Popova et al. Content and fatty acid composition of different fat depots of lambs receiving fish oil supplemented diet. Bulgarian Journal of Agricultural Science. 2008. 14(1):100-107.
It's suet, the harder fat around the abdominal organs. I'm not sure why it has a different texture. It might be more saturated.
This thread leads me to believe other people also have a harder time digesting it. In fact, now that you mention it, I think I remember some folks on the zero-carb forum reporting heartburn from pemmican made with suet, which is a shame, since it stays hard better.
Sounds like you are talking about cholesterol which is a waxy fat. I personally have no issues consuming cholesterol. My body digests it easily just as it digests other fats. Most of the cholesterol in the body is actually created by the body itself so, despite what ill informed doctors like to say, actual dietary cholesterol intake has only a minor influence on overall body cholesterol levels. Many healthful foods, like organ meats, are high in cholesterol.