"a cross between a tarantula and a jellyfish".

No one knows what ETI will look like, but UGLY is my best guess, with TERRIFYING a close second. They'll probably feel the same way about us. In fact when you think about it, most creatures on earth look ugly to us; even the ones we think are exquisitely beautiful, like butterflies or nudibranchs, would lose their appeal if they were the size of Shetland ponies. The only animals we usually think of as pretty are mammals and birds, and if you remove their fur or feathers they quickly become grotesque gargoyles.

I can imagine an alien description of a human:

"It is a smooth, naked creature, with patches of bristles scattered about its body. The skeleton is internal, and the musculature can be discerned squirming under the skin. Fortunately for us, most of their bodies are covered by a soft artificial armor (probably ceremonial, since it clearly provides little protection from weaponry or environmental conditions). The auditory sensors are like fins attached to the shortest one of five protuberances emerging from its trunk, and its optical organs are glistening wet spheres rolling in deep holes in that same extremity. Manipulative organs located at the ends of the shorter limbs are terrifying and seemingly clumsy, although they manage tools quite effectively with them. The feeding organ is also used for sound communication, and consists of a quivering gash in the skin surrounded by slimy pulsating flesh and bones which protude into the cavity. The sexual organs are highly differentiated in the two sexes. one carried externally, the other concealed in an orifice (see illustration) although they must really be seen to be believed. When negotiating with these hideous creatures, we should keep in mind that we must appear as strange to them as they do to us, and we should avoid flashing our (?) or extruding our (?) in disgust or exhibiting other signs of revulsion, as it may be insulting or stressful to them."