GLORIOUS ARTHROPODS!
...Er, I mean, metagame trends. Nah, I mean glorious arthropods.
Last gen, when I was most active, we had two dominant Bug-types very commonly seen in OU (Scizor and Volcarona) and not much else. Both were VERY threatening. Scizor was consistently one of the top Choice Band users throughout Gen 5 OU, able to do well on almost any team (especially on rain teams, since its Fire weakness was mitigated somewhat in the rain) with little or no support. Swords Dance sets maintained some presence throughout the generation, but didn't really start to pick up until late BW2, when someone came up with the idea of running Flying Gem and Acrobatics to smack SD Scizor's usual counters (e.g. Jellicent and Tentacruel) on the switch and that became popularized. Volcarona required Rapid Spin support and something to deal with Terrakion and Keldeo, but with those conditions met, it was very dangerous. Give it one free turn to Quiver Dance, and you'd be facing down a sweeper who could outrun nearly all un-Scarfed 'mons, tank a surprising number of attacks (especially on the special side), and blow massive holes in the opposition with surprisingly good STAB coverage. Volcarona also had the capacity to benefit from both sun and rain, unlike most Pokemon - sun boosted the power of its Fire attacks to insane levels, and rain gave it a chance to smack things around with Hurricane. Volcarona had two primary sets in BW2 - a bulky QD set aiming for longevity and relying on two moves for its coverage (the final slot was taken up by Roost), and an offensive QD set similar to the one that originated back at the beginning of the generation (albeit with the addition of Giga Drain to smack Water- and Rock-types around). Sadly, the rise of Flying-type priority over the generation shift caused Volcarona's usage to plummet.
Nowadays, the ranks of OU-viable Bug-types are a little more diverse, despite the proliferation of Pokemon who can hit them for supereffective damage and/or resist their STABs (in fact, Pinsir and Mega-Scizor have adapted by not even CARRYING Bug STAB, with the exception of U-turn on some Scizor sets). Volcarona, while it struggles a bit in the modern metagame and has become very uncommon, is still viable if you can reliably take out Talonflame, Azumarill, and fellow Bug Pinsir. Scizor is still a threat to prepare for, but its primary sets have changed pretty radically since BW. Choice Band Scizor is pretty rare nowadays; most of them are running Scizorite. Mega-Scizor's got enough power that it can afford to invest most of its EVs in bulk and still have decent offensive presence, and it's turned into a tank capable of performing multiple roles very well. It's one of the few Defoggers who don't really fear Bisharp, and it can run a great bulky Swords Dance set; both sets enjoy the XY Knock Off buff immensely. (And just think - we thought it would be lackluster at the beginning of the generation!) In terms of new additions, we now have Pinsir, Scolipede, and Heracross to work with. Mega-Pinsir has a bit difficulty setting up a Swords Dance, but its fantastic power and coverage with Earthquake and Aerilate Return/Quick Attack make it a very threatening sweeper once it's achieved that goal, and it's often paired with Talonflame (and occasionally Staraptor) to form a Flying-spam offensive core. Said core has fluctuated in usefulness somewhat as the meta developed, but it's still definitely solid. Scolipede, while it didn't get a Mega like Pinsir did, was unquestionably buffed just as much - its increased Attack and acquisition of Speed Boost as a Hidden Ability allowed it to fill several roles very well. The all-out offensive set made a potent late-game cleaner and Fairy killer, it's one of the few viable Toxic Spikes users in the tier, and its beautiful combination of support qualities almost singlehandedly made Baton Pass teams, viewed as a ridiculous gimmick last gen, into such a potent force that they needed to be nerfed TWICE. Heracross, terror of RSE OU, is the last of the influential Bugs this gen (well, there's also Genesect, but that got the boot a while back). At the beginning of the generation, a lot of people were skeptical about the potential of its new Mega; they saw bad Speed and a 4x weakness to the near-omnipresent Flying attacks in OU and dismissed it out of hand. However, Mega-Heracross definitely has quite a bit going for it. The first thing to note is its ridiculous power; it has the highest base Attack stat in the entire game and is capable of 2HKOing nearly everything that isn't Ghost-, Fairy-, or Flying-type with its STABs alone. (To give you an idea of how strong 252+ Atk Heracross is, Pin Missile gets a clean 2HKO on Quagsire and Close Combat does the same to Skarmory.) Accordingly, it's right up there with Black Kyurem as one of the most potent stallbreakers in the tier. It's even got Rock Blast to deal with Gengar and Flying-types on the switch, although it still hates facing them 1v1. Mega-Heracross has enough natural bulk to tank a surprising number of attacks if EV'd properly, although lack of a reliable recovery option holds it back from acting as a dedicated tank to an extent (c'mon, Game Freak, you give Roost to Illumise, but not a Bug who can actually make good use of it?). Now, this isn't to say that everything's great for Mega-Heracross - base 75 Speed is unfortunate, it requires teammates who can eliminate Flying- and Fairy-types to be most effective (and Aegislash, if you choose to forgo Earthquake for Swords Dance), and it's very susceptible to status - but it still murders stall teams almost singlehandedly and can do a number to offensive teams once Talonflame and company are removed.
Honorable mention goes to Galvantula, who got a major buff in the addition of Sticky Web to its movepool, but struggles to be effective all the same. If it had a bit more offensive presence or decent bulk, it'd be a threat, but as it stands, you're unlikely to see a competent player running Galvantula in OU (man, if there was ever a Pokemon who deserved a Mega...) for the time being. It's still the cutest FE Pokemon, at least.
