Beer is most commonly made from barley malt, water, hops and yeast, which means it's usually vegan. You're most likely to find isinglass, gelatin, glycerin or casein in non-vegan beers and other alcoholic beverages, but some wines, ciders and beers can also contain milk, eggs and honey.
Guinness is 100% vegan – animal products have not been used either as ingredients or filtering agents since 2018. Prior to this, a pint of the dark stuff wasn't considered vegan; this was because it used isinglass, a substance taken from fish bladders, to make it clearer.
Plant Based News notes most beer is in fact vegan, as it is usually made from barley malt, water, hops, and yeast, which are all plant-based ingredients. Some beers that are not considered vegan use animal products, like isinglass and gelatin, in the brewing process to make the color of the beer clearer.
The short answer: In general, most beer is made from barley malt, yeast, hops, and water. These ingredients contain no animal products and therefore make beer both vegetarian and vegan friendly.