Ekans certainly is a snake, isn’t he? He’s got all the obvious characteristics – from bands about the throat to the slitted eyes to a little rattler. The official art makes him look ready to swallow an egg, too. Pretty recognizable mish-mash of generic snake parts as he goes, which ain’t a bad choice. He shows up pretty early and the obvious motif helps players associate the color purple with the poison-type. Or it would, if Ratatta hadn’t already shown up, or if the somehwhat-arbitrarily Poison-type Bulbasaur hadn’t already helped throw that off. Grr.
Arbok is a bit more interesting – it drops the grab-bag approach for taking a hardline “Indian cobra” stance, and pulls it off pretty well. Even got a forked tongue now! He’s thicker, more powerful, got visible teeth (though not fangs), gained angry markings (more on those later)… he’s like Ekans, but more ferocious and elegant at the same time. Great grow-up!
By the numbers, Arbok isn’t too great at anything in particular, but he gets early access to poison-type moves in a package that’s less likely to crumple when hit as bug-type, which can make him useful. He’s not particularly bad at anything, either. Some useful abilities like a shot at Shed Skin and the guaranteed-paralysis Glare make him a solid choice for lay on status effects while having a bit of staying power.
Being one of the major antagonists in the TV show for the first five years, the Ekans line is definitely very recognizable as a figurehead for the Poison-type, which means he gets a lot of promo time and makes him less disposable of a design. Good for him, finding his exposure!
Ekans and Arbok have actually gone through some notable design changes over the years – Ekans started out with more yellow bands around his body, but Arbok is the interesting one. There have been at least three distinct sets of markings shown on her “hood” across the games, with at least two others shown in other media; he even used to have stripes across the back of his hood way back in the day. This definitely fits with the Pokédex description of the species having different marking patterns in different regions – unfortunately, we haven’t seen a new variant since about 2004, which feels like a real missed opportunity that modern games could and should capitalize on.
Quick sidebar, literally every single translation of Arbok’s name is give-or-take the same name. Arbok-Arbok-Arbokas-Arbokuài-Arbok. Don’t see that much outside of Pikachu.
Other than that… he’s a venomous snake who does snakelike things. What more do you want? He’s also the snake, with only one major competitor to his throne in the last two decades, and is generally a pretty prolific and recognizable Pokémon in his own right. As a such a recognizable Generation-One Pokémon, he’s probably a Must-Have.
Any and all appreciation for Ekans and Arbok is welcome in the comments!