Alien Life Forms (Long War)
Much like in vanilla, aliens deploy in "pods", which are "activated" by seeing them. Unlike in vanilla, alien pods are frequently made up of several different types of aliens, such as a Thin Man leading some Sectoids, or some Mutons with Floater scout.
One alien in a pod will be a squad leader, with improved stats and sometimes new abilities. They can almost always be identified by their higher health than normal.
Sectoid
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Sectoids behave in much the same way as you'd expect from vanilla, with low Mobility, HP, Aim, and Will. Basic Sectoids have access to a few more psionic powers than before, being able to Mind Merge, Mind Fray and Psi Panic. Their low Will means they won't be particularly effective at using these powers (Mind Fray will typically only do 1 damage for example). Be aware that Mind Merge has been altered so it does some damage to the merged target if the merger dies, but doesn't kill outright. Sectoid squad leaders get the Low Profile perk.
Drone
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Drones will be encountered right from the beginning of the game, even in the first mission. Drones behave similarly to vanilla, but, like most mechanized units, have some damage reduction. Their constant light cover from flying with damage reduction and ability to mutually repair can make them extremely frustrating to kill.
Outsider
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Long War Outsiders are enormously more dangerous than vanilla Outsiders. They have 9 HP or more and regenerate 3 HP per turn, meaning it will take a concerted effort from the whole squad to take them down early game. Like vanilla, Outsiders are only found in charge of UFOs, usually holed up on the bridge. You should be careful not to activate a UFO's Outsider until you're ready for it, since their HP regen means they can easily take on 1 or 2 XCOM soldiers without even taking damage.
Thin Men
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Thin Men are quite similar to vanilla, with high mobility, mediocre aim and low Will. The main change is that their poison has been replaced with Acid, which lowers aim and mobility as poison does, and affected units take slightly more damage from attacks, but acid no longer causes damage directly. Instead, acid inflicts damage to a victim as they take actions, so moving, reloading, firing, overwatch, etc will all cause an affected unit to take 1 damage. Hunkering Down is safe and a unit that simply remains still and hunkered for the duration of acid will take no damage at all.
Floater
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Floaters behave identically to vanilla floaters. Floater squad leaders gain the Damn Good Ground ability, which, given their ability to fly, makes them significantly more dangerous than regular floaters.
Chryssalid
Chryssalids are similar to vanilla Chryssalids. A big difference is that Chryssalid squad leaders have Lightning Reflexes, which can seriously reduce the amount of damage a charging Chryssalid squad will take from overwatch fire if the squad leader moves first.
Seeker
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Seekers can be found both in pods made up entirely of Seekers, and accompanying squads of other aliens. This indirectly makes them more dangerous than vanilla, since a Seeker itself isn't much threat, but a Seeker supporting other aliens can be a deadly distraction. Long War Seekers are otherwise similar to vanilla Seekers, except that as mechanized units they have some damage reduction, which makes them difficult to damage with low power weapons like pistols and carbines. Note that Battle Scanners are available to any soldier as carried items in Long War, giving an early counter to Seekers. Additionally, they only cause 1 damage at end of turn when they start their strangle (as opposed to 2 in EW: and with the +1 HP of the starting armor, would normally mean time in the infirmary), to compensate for the fact that an Overwatch barrage has much less chance of killing the Seeker when it reveals itself. However, they do now have the habit of going into Overwatch while in stealth. If you savescum, and a soldier has an AoE attack handy, you can use this to your advantage if you trigger this.
Muton
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Mutons are similar to vanilla Mutons, and will start with Plasma Carbines and later upgrade to Plasma Rifles. Muton squad leaders have significantly more HP than regular Mutons, and have the Opportunist perk.