White Park cattle have been at Dinefwr, near Llandeilo, since 920 and were referenced in laws of Hywel Dda.
But there are now only 1,500 breeding heifers left in the world and the Carmarthenshire herd needs a new bull.
Quattro the bull has arrived on Wednesday, following a National Trust fundraising campaign.
White Park cattle are characterised by their pure white coats and black noses and ears.
The breed helps to manage the estate at Dinefwr, providing a natural and effective method of keeping wildflower hay meadows at bay.
Quattro will be responsible for serving a herd of almost 30 females and their offspring.
Rhodri Thomas, who will take the reigns as head stockman, said: "The arrival of a new bull has been highly anticipated.
"To sustain the Dinefwr line, we were searching for a pedigree White Park bull and we are pleased to say we have found that in Quattro.
"It will take him up to four months to serve the whole herd and we'll find out how successful he's been next spring."
The young bull is from a herd of White Park cattle bred in Devon but his family history lies in Dinefwr.
National Trust launched a campaign to save the breed's bloodline last year and is now halfway to reaching its Â£36,000 target.
The Prince of Wales, who has been president of the charity since 2003, made a donation to the Save the Herd at Dinefwr campaign during a visit in February.
A Clarence House spokesman said Prince Charles was "really pleased" to learn Dinefwr had "finally got a new bull to help continue the herd's development and breeding success".
A bull billed as the saviour of a rare cattle breed which has roamed a Carmarthenshire park for 1,000 years has arrived.