A type of lake/sea cryptid also called the Long Neck, Merhorse, Water Bull, or Kelpie.
For decades, some cryptozoologists have held that certain sightings of "sea monsters" are not the typical Plesiosaur-style reptiles, but actually long-necked seals. For years I thought this was a horrible idea- I couldn't imagine a seal that would look anything like a Plesiosaur, and felt like it was silly to just make up an animal on-the-spot to introduce a new theory.
After rereading Coleman and Huyghe's Guide, though, this idea stuck with me more. A lot more. First of all, there are actually quite a lot of reports of these Water Horses. Not just "this plesiosaur-like cryptid could've actually been a seal", but stuff like "the animal had slick fur", "it had visible whiskers", etc. The areas in which these cryptids are reported are worldwide, but they're mainly seen in the UK and off the coast of Africa. The descriptions of appearance and behavior are actually really consistant, and it even sounds like there are two kinds- sexually dimorphic males and females of the same species, maybe.
Also, the bit about the long-necked seal being totally made up? I was wrong there too. Acrophoca was a seal that lived in the Miocene and went extinct in the early Pliocene; it possesed a (comparatively) long neck, with a longer snout than modern seals and an elongated, almost serpentine body. Interestingly enough, it was the prehistoric ancestor of the leopard seal- a modern pinniped which is known for it's very reptilian appearance.
So this is my take on the long-necked seal, or Waterhorse. A relative of Acrophoca and the Leopard Seal, with deceptively reptilian features and a long neck. Just going off oceanic sightings, this could be a cryptid all on it's own- though it's also been proposed that Morag and the Loch Ness Monster, among others, may be a freshwater offshoot.
Do I subscribe to this theory now? Eh... not 100%, not yet. It makes sense, but it's not bulletproof. The main thing is, why don't they appear out of the water? There are land sightings of some Waterhorses, mainly the supposed land-locked ones, but they're pretty rare. Wouldn't a twenty foot seal, be it oceanic or freshwater, be coming onto the shore a lot more often than that? Still, it's at least something to think about.