I don't think you would be fined for buying the pony without the passport, the onus for obtaining the passport and making sure all legalities are adhered to lies with the person selling it, they would be fined for breaking the law, not you.  Buying a pony without a passport is not illegal, but selling one without a passport is.  That said, you have been ill advised going ahead with the purchase without the passport being in place but if you are sure of the ownership of the pony i.e. was it in its last home for a long time etc then again nothing to worry about, the deed is done.  Now you must obtain a passport straight away and I would recommend the horse passport agency.  I have never had to give a passport number to obtain horse insurance but they will ask for the microchip number if there is one (recommended) and may also ask to see a vet certificate so you might need to arrange for a vetting if you haven't had one done, the vet will also have to complete part of the application form for the passport as there is an identification diagram of the horse which must be completed by the vet and it would be a good time to get vaccinations done and teeth checked at the same time.  Try not to worry, the deed has been done and now you need to put things right.