The Dire rat can be seen as merely a pest when alone, but when in a large group these Dire rats will tear apart party members that use melee attacks. The part about them that makes them so frightening is their filth fever, whenever the Dire Rat lands an attack on a PC, the PC contracts this fever and loses 1 healing surge, if they fail to resist the disease they take a -2 penalty to AC, fortitude defence and reflex defence. Fail again and they take another -2 penalty to those stats but then also lose ALL healing surges and lose the ability to heal. Denying the PCs the ability to heal can easily screw them over, using a group of these Dire Rats can in-fact do that and it will cause a lot of trouble for parties.
The Unicorn can be quite a formidable foe, and when used as a mount for a NPC the party needs to defeat it will cause all kinds of issues. The Unicorns attacks are pretty generic, it can charge you for nasty damage, kick you and even push you around when it charges. But every Unicorn has magical properties and these properties allow it to teleport, automatically cause a nearby ally to spend a healing surge and make itself immune to a party member while also tricking that party member into attacking his/her ally. Dusk unicorns especially can move people around without touching them and it can even use its teleport to directly move someone around instead of just teleporting itself.
Chokers are fairly generic, they grapple a party member and then they try to choke them, the issue with this is they have an immediate passive where they use their grappled victim as a body shield. So the only way to effectively kill the choker is for the choked party member to keep hitting it, or by having more than 1 other person focus it down, despite being able to only use this ability once per round, the choker can recharge it with every choke or basic attack. If you break the chokers grappled, it will go into stealth with it's chameleon like ability and attempt a grapple again next round. Chokers can easily cause distress and panic among an encounter if a party member gets grappled.
These humanoids have the ability to appear as any person of any race, while unable to copy apparel and accessories Doppelgangers can effectively take the throne of a king without ever drawing a blade. Because of their sneaky nature, the Doppelganger can easily blackjack a party member and disguise themselves as that party member and turn the party members against eachother easily or lure them into traps and danger. Doppelgangers are masters of disguise with ther unparalleled abilities of disguise and stealth, and can be an extremely large loophole in a story plotline.
Lamias, much like the doppelgangers can change their form, however they prefer to guise themselves as a beautiful woman and lure men to their lair where they consume him alive. Lamias are actually an entire swarm of scarab beetles that are wrapped around a skeleton of a powerful fey creature. Because of the nature of the Lamia it can be relatively hard to locate and even when located it can retreat through small spaces and cracks, the Lamia can easily come back to attack the party most likely by isolating one of the members.
Gonna upload another one later today. Gonna need some requests/suggestions, Monsters ones I prefer since they're the easiest and less time consuming.