A slight variation with this thread - creatures that sound like other critters ( or just plain odd).
The elusive Grasshopper Warbler.
Here's an entertaining account from 1768 of the bird by the famous English naturalist Gilbert White from his book the 'Natural History of Selbourne':
"The grasshopper-lark began his sibilous note in my fields last Saturday. Nothing can be more amusing than the whisper of this little bird, which seems to be close by though at a hundred yards’ distance; and when close at your ear, is scarce any louder than when a great way off. Had I not been a little acquainted with insects, and known that the grasshopper kind is not yet hatched, I should have hardly believed but that it had been a locusta whispering in the bushes. The country people laugh when you tell them that it is the note of a bird. It is a most artful creature, skulking in the thickest part of a bush, and will sing at a yard distance, provided it be concealed. I was obliged to get a person to go on the other side of the hedge where it haunted, and then it would run, creeping like a mouse, before us for a hundred yards together, through the bottom of the thorns; yet it would not come into fair sight; but in a morning early, and when undisturbed, it sings on the top of a twig, gaping and shivering"
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Another secretive bird - the Water Rail. Sounds like a squealing pig:
I well remember some years ago hiding in a willow tree for nearly 2 hours on the edge of a muddy marsh trying to catch a full view of an adult bird with its young in the open. I knew birds were present because of the continuous contact calls - I wanted to confirm breeding in my district as it was a scarce local breeder. Eventually, one adult and 2 half-grown chicks emerged from behind the sedges with mud trailing behind their legs - the filthy swines.
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Little owl (to me this has always sounded somewhat similar to a small dog yapping/yelping in the distance):
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Completely different category - a Scops owl -it has a totally weird call, sounds almost like a radar blip.