I'm finding I'm doing a bad job of introducing my PCs to new monsters.
As you make your way through the thick pine forest, the wind whistles above you. Ahead you notice some trees swaying more than the wind should be causing them. Then you feel it. The ground shudders. Several ferns nearby tremor, shaking dew droplets cascading over the gnomes and halflings in your party. Then again. Some thing big and heavy is out there. Peering into the dim light of the forest, you suddenly see a tree trunk hurtling toward you. You're able to scatter to the side before it hits anyone, but you have no time to rest before the monstrosity that threw it is upon you.
Roll initiative!
The monster is massive, standing above the tallest of you by about four feet, with almost no hair on its pale scarred skin. Its lower jaw is filled with sharp snaggleteeth protruding out at odd angles, and its cruel eyes glimmer with only the barest intelligence. Primitive bone, leather, and leaf fetishes cover its lower torso and forearms. With an inarticulate growl, it uproots a young pine before your very eyes and attacks with unusual fury.
I'm finding I'm doing a bad job of introducing my PCs to new monsters. I don't feel like they get a good picture of what they're facing. Any tips for introducing an encounter in a interesting and descriptive manner?
Say "Hi everyone, this is Randy, he's a beholder but try not to hold that against him."

(Dungeons & Dragons)
Rulebook featuring "high magic" options, including a host of new spells.