Why are some beers non-vegan? Some beers (for example, Guinness) are marked as 'not suitable for vegans'. What is in the beer such that this is the case?
There are two main ingredients in beer that might cause them to be unsuitable for vegans. The first one might be fairly obvious. Honey is sometimes used as a sweetener, especially in meade, but certainly in other beers.

However, the main cause is a fining agent called Isinglass that is made from ground swim bladders of fish.