Hot Water bottles were used as early as the 16th Century! Early examples of hot water bottles were not bottles at all but containers which would be filled with hot coals from a fire and place in a bed before it was used.
Designs then progressed to using hot water for warming. The containers were made of metal or wood and were often wrapped in a cloth bag so they could be used in bed with the occupant.
Modern Rubber Hot Water Bottles were invented in 1903 an were manufactured from natural rubber or PVC. The design for hot water bottles was patented by a chap called Eduard Penkala, an Croatian inventor and called ‘the Thermofor’.