(Dorji Wangchuk’s personal blog for pure speculations and reflections)
Don't hate me or quote me on it, but I think they are the beings ridden by the Nereids of Greek mythology. They're called ichtyocentaurs or hippocamps, and they appear a lot in Gandharan art. Actually, I don't think this argument would make all that much sense, since makaras were busy doing their work at the same time in Mathuran art. Did you hear that the Loch Ness Monster has been 'converted' recently to be a protector of Buddhism? It's also a 'water worm.' What is the Sanskrit root of Makara, btw? Some think Makaras are dolphins (fresh water dolphins that are used to live in the Ganga), but I think they're more likely manatees (or a closely related species), if you want a sea creature that's still around and findable in nature.
And there is a water horse, འཆིབས་སུ་སྤྲིན་གསེབ་ཆུ་རྟ་སྔོན་པོ་བཅིབས། ridden by A phyi chose kyi sgrol maMakes me think of hippopotami.