Many of the common monsters in the first 2 tiers of 5.0e are Intelligence 9 or lower and are described as warlike, violent, chaotic, rough, or cowardly and have Charisma under 10.
First, PCs do not get LTH until fifth level, at the earliest. And that is at the expense of a possibly, more threatening spell. That means we are talking about higher CRs than tier 1. But please, let's have a look at the MM:
AArakocra (CR 1/4)
Azer (CR2)
Banshee (CR4)
Spectator (Beholder Light) (CR3)
Bugbear Chief (CR3)
Cambion (CR5)
Couatl (CR4)
Shadow Demon (CR4)
Imp (CR1)
Spined Devil (CR2)
Doppleganger (CR3)
Black Dragon Wyrmling (CR2)
Blue Dragon Wyrmling (CR3)
Green Dragon Wyrmling (CR2)
Red Dragon Wyrmling (CR4)
Brass Dragon Wyrmling (CR1)
Bronze Dragon Wyrmling (CR2)
Copper Dragon Wyrmling (CR1)
Gold Dragon Wyrmling (CR3)
Silver Dragon Wyrmling (CR2)
Dryad (CR1)
Duergar (CR1)
Drow (CR 1/4)
Drow Elite Warrior (CR5)
Faerie Dragon (CR1)
Flameskull (CR4)
Flumph (CR1/8)
Ghost (CR4)
Ghast (CR2)
Githyanki Warrior (CR3)
Githzerai Monk (CR2)
Gnoll Fang (CR4)
Deep Gnome (CR1/2)
Goblin Boss (CR1)
Grell (CR3)
Green Hag (CR3)
Night Hag (CR5)
Sea Hag (CR2)
I will stop now. These are all the monsters in the 2014 MM up to the letter G. All of the ones I listed have a CR rating below the group's expected level for combat.
And all of them have an intelligence above a 10! It is literally not that hard to see with this
evidence that these creatures are intelligent enough to make life miserable for the group once they come out of LTH.
And, as a side note, if we averaged the intelligence of the monsters above, they would have a higher or the same intelligence than the stat blocks of a knight, druid, cultist, cult fanatic, guard, bandit or spy. Go to a keep full of knights, kill half of them, cast LTH, and see what they come up with when you pop out. That is the same as
all of the creatures above.