The name Ekans is a play on what it is... the word Ekans backwards reads 'Snake'. Arbok works in the same way, only it is 'Kobra' (cobra) backwards. Aabo backwards is Boaa (boa).
The Japanese version of Pokémon had Ekans under the name Aabo (also spelt Arbo) and Arbok kept its name, Arbok.
The proposed English name for Ekans which was abandoned, was to keep it as it had been, Arbo. I don't know why this was ditched in favour of Ekans. Perhaps Aabo and Arbok were too similar.
It's true that in Japanese, the names have more difference between them than they would in English. Arbo is pronounced 'aa-bo' while Arbok is 'aa-boh-ku'... an English pronounciation of Arbok does not have the 'ku' sound on the end.
The Japanese characters used in the writing of the Pokemon's names is called katakana. (There are two other Japanese 'alphabets'/sets of characters, called hiragana and kanji.)
Arbo, in Japanese, is actually spelt Aabo, or A-bo (the dash means the a sound is elongated) and in katakana it looks like this:
Arbok is actually spelt Aabokku or A-boku and in katakana looks like this:
Why *does* is go like that? Well, to simplify it a lot, the Japanese 'alphabets' are composed of syllables which end in vowels... for example, ku, chi, tsu, re, a, i, o, etc. The only exception to this is the letter n/m.
You'll notice if you see Japanese names that they're all composed of syllables ending in vowels (bar n/m). The Japanese names of Pokemon, even the English words (like Gallop (Rapidash) and Striker (Scyther) are written of the Japanese characters and appear as Garopu and Suturaika respectively.
So how do I know that Aabo is the Japanese way of spelling what's intended to be Arbo, and Aabokku is meant to be Arbok? Merchandise, of course. ^_^. Japanese merchandise sometimes spells the Pokemon's names using English letters, and in those cases, their names are spelt Arbo and Arbok. ^_^.
I spell Aabo as that usually instead of Arbo because I think of it as a back-to-front version of the word boa. ^_^.
Understanding the concept of katakana also explains why 'aabo' backwards is 'boaa'. Bo cannot be rearranged because it is a character complete.
Anyway, I'm sorry if I completely confused anyone who's not at all familiar with Japanese... ^_^;;