What looks like a highly marbled portion of meat, is actually a condition known as muscular steatosis. Other terms include “steatosis,” “callous,” “calloused lean,” “calloused ribeye,” “callus,” or “woody callused.” It occurs when muscle damage or nerve degeneration happens and fatty tissue permeates into the muscle tissue. In extreme cases, the fat can completely take over the muscle and become solid fat.
This condition most often occurs because the animal was injured at some point in its life. Other sources of this condition include vascular abnormalities, biopsy locations, or when animals rear up on their hind legs.
While it is still safe to eat, the quality will be very poor and tough.
Sources: Dr. Liz Boyle and Dr. Terry Houser, K-State Research and Extension Meat Specialists; https://meat.tamu.edu/2013/05/06/muscular-steatosis/