Dragapult, the Psuedolegendary of this generation. While previous Pseudolegendaries are all bulky in some way, shape or form, Dragapult goes the other direction and is primarily a Glass Cannon having "poor" defenses and a myriad of weaknesses in contrast to useable offenses and an insanely blistering speed that makes it the 5th fastest pokémon in the current game (and 14th fastest if we go full national Dex) and the best of both movepools from both its Dragon and Ghost typings. Dragapult is a threat that must be accounted for when team building lest it run over your team.
Sabotuer
Dragapult
Item: Heavy Duty Boots
Ability: Infiltrator
-Will-o-Wisp
-Hex
-U-Turn
-Dragon-Darts/Dragon Pulse/Thunder Wave
Nature: Timid (+SPE, -ATK) or Modest (+SATK, -ATK)
EVs: 252 SATK/252 SPE/4 ATK
Dragapult flexes its Ghost typing with this moveset, spreading status as the situation calls for it (Will-o-Wisp for physical attackers, Thunderwave is an option for speedy threats) and then slams crippled targets with a Base 130 STAB Hex. U-Turn allows Dragapult to scout out a suspected switch or coverage move while final move is up to preference. Thunderwave as mentioned before is great for speed control, but your team may already have options for that in which case a Dragon STAB move (Dragon Darts so you aren't bait for the pink Blobs, or Dragon Pulse for more consistency is speccing your EVs) offers great neutral coverage.
Good teammates include others that can spread status (i.e. Toxic and Toxic Spikes) that make good pivots for U-Turn (Toxapex resists Ice and Fairy moves, spreads Toxic, and enjoys Dragapult crippling physical attackers and scaring off Psychic types) (Mandabuzz resists Ghost and Dark and spreads Toxic) (Togekiss resists Dark and Dragon and enjoys the T-Wave support).
Infiltrator is preferred ability, bypass Substitute and Screens. Clear Body is not as useful in Singles and while Dragapult can abuse Cursed Body due to its typing, it is too frail to take many physical hits.
Other Options
Light Screen/Reflect -Dragapult is a good Dual Screen user, but let's face it, Grimmsnarl is far better.
Substitute -Dragapult tends to attract Thunderwave like furniture attracts cat hair, so Substitute is good for shutting down most attempts
Doubles Options
Clear Body -better in Doubles due to the number of Intimidate users running around, though Dual Screens/Aurora Veil is still a powerful strategy over there.
Ally-Switch -Dragapult has a few immunities and good speed to abuse Ally Switch
Checks and Counters
As fast as Dragapult is, there are several things in the game that are faster. VS a Neutral Nature Max Speed Invested Dragapult, anything that is at +1 Speed (Scarf, Dances) with max investment and at least 80 Base Speed (Neutral Natured) or 68 Base Speed (Positive natured) can outspeed it. Zeraora naturally outspeeds Dragapult and can maim, if not OHKO it with Knock Off (special sets don't have an answer though). Paralyzing it works well, though watch out for Substitute. Really any speed control on a decently fast pokémon enables you to get the jump on Dragapult. Just note that you can't trap it and need to keep that answer alive, lest Dragapult gouge holes out of your team without it.
Countering every Dragapult defensively is near impossible -its movepool is far too varied and can easily run sets all over the spectrum, though Dragapult is lacking in power and/or coverage depending on the set. Grimmsnarl is a good general answer resists Dragapult's STABs, can Paralyze it with a Prankster T-Wave, can reliably OHKO with a STAB Sucker Punch (or just about any STAB) , but is 2HKOd by most coverage moves (and can be OHKOd by Steel Wings) and getting Burned or T-Waved will ruin Grimmsnarl in Turn.
Physical sets can be shutdown by physical walls (Corviknight and Skarmory beware of the occasional Fire Blast) while Special sets can be stymied by defensive Primarina, Clefable, and Blissey.