I wanted to draw your attention to a couple of log entries from Halley that have left people mystified.
He was approximately 430km north west of South Georgia when he described encounters with three sea creatures:
[27 Jan 1700, extract] All this Morning we have had a greate Fogg… and Sounded every two hours, apprehending my Self near Land; and the rather because yesterday and today severall fowls, which I take to be penguins, have passed by the Ship side, being of two sorts; the one black head and back, with white neck and breast; the other larger and of the Colour and siz of a young Cygnett, haveing a bill very remarkable hoocking downwards, and crying like a bittern as they past us. The bill of the other was very like that of the Crow, Both swam very deep, and allwais dived on our approach, either not having wings, or else not commonly useing them
[28 Jan 1700, extract] We have had Severall of the Diveing birds with Necks like Swans pass by us, and this Morning a Couple of Annimalls which some supposed to be Seals but are not soe; they bent their Tayles into a sort of a Bow thus and being disturb’d shew’d very large Finns as big as those of a Large Shirk The head not much unlike a Turtles.
So Halley describes:
1. a bird he takes to be a penguin with a black head and back, white breast, and a bill like that of a crow; it swims deep, dives on the ship’s approach, doesn’t appear to fly, and has a neck like a swan
2. a second bird he also takes to be a penguin, larger than the first, the colour and size of a “young Cygnett”, it has a bill that hooks downwards, cries like a bittern, and also swims deep, dives on the ship’s approach, doesn’t seem to fly, and has a neck like a swan
3. an animal thought by some of the crew to be a seal but which Halley asserts is not. It bends its tail into a bow, has large fins like a shark and a head like a turtle
What on earth are they?
My knowledge of natural history is as limited as Halley’s appears to be, though in his case we should remember he is probably seeing creatures he has no prior knowledge of and has to search for comparisons with which to describe them.
The first bird described does sound like a penguin, though the second seems more dubious. Colin Ronan suggested that the swan-like neck might refer to a king penguin as they have the “ability to stretch their necks quite considerably”,  but there’s another piece of information that appears after the voyage has ended that may make you doubt whether they’re actually penguins at all. We’ll take a look at that information in due course.
The other animal with the bow-tail, large fins and turtle’s head is very puzzling. Cook suggested a bottlenose whale, and Thrower apparently discussed the creature with a former Director of the Zoological Society of London, who proposed a bottlenose whale or dolphin.  Colin Ronan also considered a bottlenose whale or dolphin, but thought a killer whale to be the most likely. 
It surprises me that Halley doesn’t use the word ‘whale’ or ‘whale-like’ to describe the animal, as I’m pretty certain he knew what whales look like (there’s a splendid report in the Philosophical Transactions about whale-fishing in Bermuda by seamen “resolved not to be baffled by a Sea-monster”). I’m also surprised he doesn’t mention its size: an adult bottlenose whale would be nearly half the length of Halley’s ship – worthy of comment, I would have thought. It also has a small dorsal fin, whereas Halley describes his creature as having large fins. The killer whale is also of a noteworthy size, while the bottlenose dolphin is smaller, and both of these do have a larger fin – but I’m not sure anyone would describe either as having a head like that of a turtle!
If any readers with expertise have suggestions as to what Halley’s creatures might be, do please enlighten us in the comments.
 Colin Ronan, Edmond Halley: Genius in Eclipse (London, 1970), p 179
 Alan Cook, Edmond Halley: Charting the Heavens and the Seas (Oxford, 1998), p 277. Norman Thrower, The Three Voyages of Edmond Halley in the Paramore (London, 1981), p 160, note 1
 Ronan, p 179