#23 + #24: Ekans and Arbok

Quite possibly the worst named Pokemon in the games (read them backwards), you get the mono-poison snakes.

Ekans is actually rather respectable, despite low HP and not having the best speed, there is some bulk (particularly special), and the physical attack is pretty good, with special attack at only 40. You also get three abilities to choose from: intimidate (drops opponent attack by one stage, but is ignored by a few abilities and in fact reversed by contrary), shed skin (30% chance of status problem removal at the end of each turn, yet you can't be poisoned anyway) and the hidden ability unnerve (prevents the opponent from eating a held berry, which could be useful to kill chesto/resto sets).

Doesn't look too scary, so why does it get intimidate? Sure it has a rattle, but no fangs?

Since the physical attack is the best part of Ekans, this would probably be the best way to think about moves. You start off with wrap, which is low damage, but causes persistent damage and prevents switching (unless they rapid spin out of it). Early on you get access to poison sting and bite, and unless you level past when you can evolve, then there's not much worth in the level learnset other than a couple of status moves to drop opponent stats.

If you run through TM lists, you can grab earthquake, which is incredibly powerful and accurate. You also can get poison jab (powerful STAB), torment (prevents the opponent from spamming the same move), rock slide (can flinch), bulldoze (weaker than earthquake and dig, but can slow the opponent), swagger (raises their attack but confuses them, making it an interesting risk), dark pulse (a great dark-type attack but is special, so maybe take thief or payback instead) and strength, which hits quite hard and can be used in the overworld. Payback could be seen as a valid option since it hits a lot harder if used later in a turn, and since Ekans isn't too fast. You also have access to rest, protect and sleep talk.

If you breed along, you can go for things like iron tail (chain bred, very powerful but only 75 accuracy, so may not be worth the effort), both poison fang and poison tail, along with pursuit, slam, spite (I hate this since it took all of my psychic moves away in the ORAS ghost elite four dude and made it more of a challenge), the slightly risky sucker punch (which is also an underrated film) and switcheroo (swaps your item with their item, which can be pretty damn cool). You can also tutor the rather powerful aqua tail, which can wreck the super effective ground-types (if you survive it) as can seed bomb, which is kinda cool but is ruined by bulletproof, and you can also get the very powerful gunk shot, which is STAB with a nice chance of poisoning but a low accuracy.

When you evolve (or if) into Arbok, you push up to 85 attack, which is pretty good for lower tiers, the defensive stats are ok (69 physical is ok coupled with intimidate, and 79 special isn't awful), and the speed of 80, with investment, could make this a threat. You've already had access to some decent physical moves from breeding, TMs/HMs and tutoring, but sadly the levelling list is chiefly special, and since you have a paltry 65 special attack they aren't worth knowing.

Ok, now I see where intimidate comes in...

All around, Ekans and Arbok are not dreadful Pokemon, and Arbok could wreak some havoc in lower tier match-ups, but will ever threaten top end things. Poison-type is rather cool, but gets ruined by psychic and ground, so make sure you carry things to clear these threats, and you may actually need some investment in defences to survive these, other than intimidating physical ground moves. Sadly, you don't really get any great dark coverage, so be careful against psychic, and aqua tail can miss.

(All artwork presented by Ken Sugimori, taken from the Bulbapedia image archives)