Admittedly, a normal chameleon –as weird as he looks– isn't particularly scary. In fact, I think they're kinda cute in an alien-meets-dinosaur kinda way. The giant variety a la Moldvay, is a different kettle of lizard.
Lizard, Giant Horned Chameleon (from Moldvay)
No. App: 1d3 (1d6)
Move: 120' (40')
Attacks: 2 (bite, horn)
So stat-wise, it's no great shakes. Their morale would indicate a skittish creature. The best things you can say about it are that its AC is decent and 5 HD is too much for a Sleep spell to take it out. While the book's listing doesn't specifically state it, in the real world chameleons are gifted climbers, so I would probably allow a certain amount of clinging to rough walls, etc. Where the GHC really shines is in the details, though. Let's look at a few tidbits:
"The immense 7' giant horned chameleon can surprise its prey, despite its bulk, due to its uncanny ability to take on the color, appearance, and texture of its local surroundings. A victim will be surprised on a surprise check roll of 1-5 on 1d6."
Yup. That's right. You have a less than 17% chance of not being surprised. That means free attacks, and with up to 3 of them in a random encounter, that could be six attacks before you can do anything about it.
"The giant horned chameleon has three special attacks. The first is its sticky tongue, which can lash out to a distance of 5'. If this attack succeeds, the victim is immediately yanked back to the chameleon's mouth and bitten without the need for another attack roll, for 2d4 hit points of damage."
Five feet isn't exactly longbow range, but it doesn't need to be adjacent to you either. The automatic bite is a nice touch, and 2-8 points of damage is respectable enough.
"The second special attack available to the chameleon is its large horns, which inflict 1d6 hit points of damage."
Not really all that special, but still a solid melee attack for close-fighting.
"Lastly, the chameleon may, on a successful attack, knock down an opponent by lashing out with its tail. The opponent may not attack the same round this occurs."
That's a fun wrinkle for a fight: the lizard(s) just making folks lose their turn over and over. There's no save against it, and a 5 HD critter is hitting often enough. What's particularly interesting about that is since they get two attacks per round, they can ruin a PC's turn by knocking him flat and still attack for damage (bite or impale).
I could see a party wandering into some large room that was a chameleon lair, housing 5-6 of these things and having a pretty interesting encounter.