When Mahārāja Bharata wanted to enter the forest to collect kuśa grass, flowers, wood, leaves, fruits, roots and water, he would fear that dogs, jackals, tigers and other ferocious animals might kill the deer. He would therefore always take the deer with him when entering the forest.
Srimad Bhagavata Purana
श्रीमद्भागवतमहापुराणम्
मूल श्लोकः
कुशकुसुमसमित्पलाशफलमूलोदकान्याहरिष्यमाणो वृकसालावृकादिभ्यो भयमाशंसमानो यदा सह हरिणकुणकेन वनं समाविशति
kuśakusumasamitpalāśaphalamūlodakānyāhariṣyamāṇo vṛkasālāvṛkādibhyo bhayamāśaṃsamāno yadā saha hariṇakuṇakena vanaṃ samāviśati
SB 5.8.12