The word ‘ambulance’ hasn’t always referred to a vehicle.
Around the time of the Crimean War, (think ‘Charge of the Light Brigade) the French set up temporary movable hospitals close to the battlefields, called ‘hôpital ambulant’, literally ‘walking hospital’. (Ambulant coming from the Latin verb ambulare, meaning ‘to walk’).
Later, the same word was used to describe the wagons used for transporting the wounded to the field hospital.
In time, the word hôpital was dropped, and ambulant changed to ambulance.