The Helena Manatee, also known as the Manatee of Helena is a supposed species of manatee from the island of St. Helena in the Atlantic Ocean, about halfway between the continents of South America and Africa. The island was supposedly populated by a large number of manatees during the days of colonization. The manatees are said to behave like seals, being semi-aquatic and coming up to land several times. There's no evidence except for eyewitnesses (of which there were only 2 known reports of), and sightings have all but ceased.
While it is entirely possible that a manatee species could have adapted to remain semi-amphibious, it is also possible that a species or rogue group of walrus was once found on and around the island. Walruses would have not been a known sight amongst travelers in the area, and their fat bodies could be mistaken for a manatee, especially if those seen were young and/or a particularly tuskless species.
With this in mind, the walruses would have had to have traveled a great distance to reach St. Helena, as they are not normally seen much further south than Scandinavia.
More likely, the creatures are possible misidentifications of either sea lions or southern elephant seals especially the latter as the species once bred on the island (see the external link).