Your equine may have been issued with a passport from a non-UK passport issuing organisation (PIO). This may be because you have imported the equine into the UK from another country, or you may have bought the equine from someone who originally registered the equine in another country at birth.
Legally, as an owner, you must register the equine with a UK based PIO if the equine is in the UK for 30 days or longer.
The UK PIO will typically "overstamp" your original non-UK passport to show that it has registered your equine and updated the CED with its details.
The sole exception to this are equines whose UELNs begin with 372414. These are registered with the UK-based Northern Ireland Horse Board Co-Op Society Ltd.
A list of Defra-approved PIOs is here. You may re-register your equine with any of these listed PIOs: