Travelling with dogs on public transport in Germany has a sort of interesting bureaucratic charm to it. In most European countries, there is a fairly simple system: dogs either travel for free or there is a low cost dog ticket. Not so the case in Germany, for that would be too easy - depending on the context they can be classified as dog, child, or adult ;-).
Local Trips in Berlin
While there are some German cities where dogs can travel free of charge, that is not the rule and definitely not the case in Berlin. Dedicated dog tickets do not exist in Berlin and a size-based rule applies: if they can fit into a carrier, small dogs (and other pets) may travel free of charge. On the contrary, large dogs must be leashed, wear a muzzle, and need a reduced fare ticket. However, if you already have a time-based ticket (day, weekly, monthly, or yearly ticket) you may bring larger dogs with you at no extra cost. While you may see people with unmuzzled dogs on the S-Bahn or U-Bahn, buses are much stricter and the driver can refuse to let you on unless your dog is wearing a muzzle!
Long Distance Travel
It should first of all be said that travelling nationally within Germany or across Europe is in principle only possible using trains, ferries, and planes. There are very few long distance coach or bus lines that allow dogs (or other pets), with the exception of replacement services for obstructed train lines. Minivans and minibuses in Eastern Europe are hit or miss and depend largely on the sentiments of the driver.
Europe by Train
If you have a small pet that can fit into a portable carrier, it can travel at no extra cost. For anything bigger (in most cases this is only applicable to dogs), you will need a separate ticket. As mentioned above, in many European countries the dog fare is quite cheap compared to human tickets and in some countries, particularly in Northern Europe, they can even travel for free. German railway operators in contrast (for intercity travel within or for international trips originating from Germany), charge 50% of adult fare.
Deutsche Bahn a.k.a the German National Railway
Until recently (February 2023), purchasing a dog ticket in Germany was both an expensive and frustrating experience. It is now possible to buy a ticket for your dog online via the new "Next" booking system and the Next DB Navigator App. Please be aware that this is not yet the default booking system of Deutsche Bahn - it is a different website address and a separate mobile app that needs to be downloaded! This means that your previous ticket purchases do not show up in this system and if you buy a ticket on the Next website, you will only see it in the Next DB Navigator app, not the DB Navigator app. This new system is also currently only available in German. For information on how the old multilingual ticketing system operates, see the next section.
Deutsche Bahn - Multilingual Website or Mobile App
Dogs can be classified either as unaccompanied minors or adult passengers. How can that be? Well, they are never counted as accompanying children, because with many types of tickets this would mean that they could travel for free (and companies certainly don't want that). On the other hand, they can be included on tickets that are meant for groups of adults such as the Schönes-Wochenende-Ticket or other regional varieties.
Furthermore, dog tickets can only be bought directly at the train station: from the ticket office or from one of the ticket machines. To be clear, this is not possible at all stations - only the ones which function as stops for intercity trains. Such tickets are non-refundable. Technically, if you want to be stubborn about it and buy a refundable ticket, you can go online and purchase a ticket for an unaccompanied minor.
Tickets for unaccompanied minors cannot be bought for international journeys. It is also not possible to buy several of the same sort of ticket in the same purchase, so that means going back and repeating each step of the process if you have more than one dog. Suffice to say, this approach is generally not worth the hassle.