The use of animal products in winemaking is notably ancient. Ox blood and egg whites in particular have a long history, where these products and other proteins have long been used for clarification and stablisation in a process known as ‘fining’.
Fining is used for a whole smorgasbord of things – including removing haze in white wines, moderating harsh tannins in red, and even removing pinking in Pinot Gris. The way it works is that the proteins bind to particles in the wine and then drop out, harmlessly, to the bottom of your tank/barrel, leaving behind a clearer/brighter/less tannic wine.
The animal substances used are broad too, including casein (derived from milk) to albumin (egg whites), gelatin and isinglass (from fish swim bladders). It’s not just weird animal body parts either – other finings products include plastic polymers (like PVPP) and bentonite (which is a fine clay), although the animal proteins remain popular as they’re cheap.
Of course, many wines could be classed as vegan as they’re not fined – especially reds which spend a long time in barrel, as undesirable tannins (and more) tend to fall out of solution over time anyway. Moreso, the actual amount of fining agents left behind in wine is almost undetectable, but given that egg and fish products in particular are considered allergens, many need to be listed on labels (which only heightens awareness).