Ask H&H: Is a German passport valid?

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Q: I bought a pure-bred Westphalian horse from an owner in the UK. I understand that the horse was imported two years ago and he has a German passport. The previous owner is not registered in the passport — only the German breeder is listed as owning the horse.

What should I do about registering myself as the new owner on the passport? Do I send the existing passport to a passport issuing office (PIO) in the UK and if so, is there one for the Westphalian breed? Is the German passport valid in the UK or do I have to apply for a new UK passport?