The word sahasrasirsa refers to the Personality of Godhead known as Garbhodakasayi Visnu. Although the Lord appeared as Ksirodakasayi Visnu, He has been described here as Sahasrasirsa Visnu because He is nondifferent from Garbhodakasayi Visnu
SB Canto 4
The great sage Maitreya told Vidura: When the demigods were thus reassured by the Personality of Godhead, they were freed from all fears, and after offering their obeisances, they returned to their heavenly planets. Then the Lord, who is nondifferent from the Sahasraśīrṣā incarnation, got on the back of Garuḍa, who carried Him to the Madhuvana Forest to see His servant Dhruva.
The word sahasraśīrṣā refers to the Personality of Godhead known as Garbhodakaśāyī Viṣṇu. Although the Lord appeared as Kṣīrodakaśāyī Viṣṇu, He has been described here as Sahasraśīrṣā Viṣṇu because He is nondifferent from Garbhodakaśāyī Viṣṇu. According to Śrīla Sanātana Gosvāmī in his Bhāgavatāmṛta, the Sahasraśīrṣā Personality of Godhead who appeared at that time was the incarnation known as Pṛśnigarbha. He created the planet known as Dhruvaloka for the habitation of Dhruva Mahārāja.