how was it named?
It is named after the research vessel
Horizon Guyot
The seamount is named after the research vessel and is also known as Horizon Ridge, Horizon Tablemount, Gora Khorayzn and . 
what is horizon guyot? A presumably Cretaceous guyot
wiki:9465917
Horizon Guyot [SEP] Name and research history
The seamount is named after the research vessel and is also known as Horizon Ridge, Horizon Tablemount, Gora Khorayzn and . During the Deep Sea Drilling Project, the drill cores called Site 44 and Site 171 were taken on Horizon Guyot in 1969 and 1971, respectively; a further drill core was obtained north of the seamount at Site 313 in 1973. This seamount is the best studied seamount of the Mid-Pacific Mountains and more is known about its morphology than any other seamount of the Mid-Pacific Mountains.