The word used is Bharatarshabha.
Bharata + Rishabha = Bharatarshabha (words combined according to Guna Sandhi).
The word means a bull of bharata race. This word is used many times in the Mahabharata to refer to Janamejaya, Yudhishtira and also Arjuna as they all belonged to the Bharata's race.
In the specific verse you have quoted, it was referring to Janamejaya.
Here's the Sanskrit Verse from Adi Parva Adi Vamshavatarana Parva section 64. The word occurs in verse 16.
एवमायुष्मतीभिस्तु प्रजाभिर्भरतर्षभ ।
इयं सागरपर्यन्ता समापूर्यत मेदिनी ॥ १६॥
evamAyuShmatIbhistu prajAbhirbharatarShabha |
iyaM sAgaraparyantA samApUryata medinI || 16||
Translation of verses 15-20 by K.S Ganguli:
And the new generation, blessed with long life, began to thrive in virtue. And thus were the four orders having Brahmanas at their head re-established. And every man at that time went in unto his wife during her season and never from lust and out of season. And, O bull of the Bharata race, in the same way, other creatures also, even those born in the race of birds went in unto their wives during the season alone. And, O protector of the earth, hundreds of thousands of creatures were born, and all were virtuous and began to multiply in virtue, all being free from sorrow and disease. And, O thou of the elephant's tread, this wide earth having the ocean for her boundaries, with her mountains and woods and towns, was once more governed by the Kshatriyas.
English Translation by K.S Ganguli from Sacred-texts.com
Sanskrit Verses from Sanskritdocuments.org
The order of the words in the Sanskrit verse is changed in the English Translation to form meaningful sentences in English.