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.