It’s actually a globosus amorphous (not “amorphous blobosus”, as I used to so confidently tell people…) — where a fertilised egg doesn’t develop properly in the womb. Instead, it tends to form a ball of fat wrapped up in skin, and feeds parasitically on the umbilical cord of its twin (you might also know someone like this).
Also known as an ‘acardiac acephalic twin’ (acardiac = without heart, acephalic = without brain) or ‘acardiac amorphous twin’, these form as a result of abnormal development of one of a pair of twins (or septuplets!). They most commonly occur in cattle, but also occur in several other species such as goats, sheep and horses.
The normal twin is usually called the ‘pump twin’ because it provides the oxygenated blood to the abnormal twin which has no heart of its own. Blood comes from the mother, via the placenta to the normal twin. It then leaves the normal twin and, instead of going back to the mother, travels through the acardiac twin. BUT the blood actually flows the wrong way around the acardiac twin as a result of this hijacked perfusion. The blood flows to the intestinal arteries first, and because the blood has already flowed through the mother and normal twin first, it is super low in oxygen. This is why, if anything develops at all, it is usually only the hind limbs and some organs — the parts that got any oxygen.