I was in the process of writing about why I don't particularly like the way that large monsters are handled in stock-standard D&D when I realized that it had gone from a semi-sane discussion of how the current rules aren't all that into full-blown complaining asshole mode.
So I scrapped the whole thing and now I'm going to talk about how I'd go about redesigning the Tarrasque.
If you've ever played 3rd edition D&D, or have ever been on a forum where it's mentioned, somebody will always mention the Tarrasque. If you've never heard of it, it's basically a nigh-unkillable giant mook monster, with good defenses, excellent attacks, and an enormous pile of hit points. It's really not very interesting, sadly, and that's my main point about it- for being an enormous, indestructible fighting machine, it doesn't really do anything. Amping everything up to eleven does not, sadly, make for an interesting monster. We know that it's not killable with regular means- did you really have to make it have 48 hit dice? Did you have to include special rules for rays and magic missiles? Was it necessary to state that it's immune to fire and poison and disease and energy drain? Did you have to list each of the totally mundane and boring skills it has? The Tarrasque is the crowning achievement of the kitchen sink design that so many detested about 3e D&D.
So, let me try and make it a little better. Couple of things going into this: This is remaking the Tarrasque so it's an interesting encounter. It's for retroclones and older systems, but you could probably port it into whatever you want. That's kind of the point, right?
The first thing that stands out to me, going from the SRD link that I've got handy, is that its health is at an absurd level. 48 hit dice is hardly necessary- even a more modest 24 is still well out of reach of any but the most dedicated party. So that's the first thing to go.
Secondly, it has an absurd array of attacks. It's able to attack with each horn independantly, as well as both claws, and a bite. This is supposed to be taking place in a fairly short combat round. It's simply undignified to have this thing thrashing around with its claws and horns and mouth all apparently attacking different things. Let's simplify, using a little math. It's got 6 attacks, with an average die damage of 11 and an average bonus damage of 10.5 per attack. So let's simplify, and say that he gets one attack per round, rolling (4d6+11)x6. The multiplication is in there because otherwise, we'd be rolling 24d6+66, and I don't know about you, but I don't have that many dice and if I did, I wouldn't roll that every round. This attack can be anything- a bite, a stomp, a massive claw, a deadly charge, whatever.
Thirdly, its immunities. It's immune to practically everything, so let's just make it official. The Tarrasque is immune to magic. There, look, done. It's almost immune to magic as it is, and at least this way, it makes it interesting- finding a way to puncture the beasts' magic immunity is now a quest-worthy goal, and makes beating it down with a lightning bolt or whatever much more interesting. This is a good place to mention its Armor Class- 30 is absurdly high. I don't care what the character optimization guys for 3e have to say- if you're going to make somethings' armor be so high that you can't hit it, just say that. Don't bother including stats for things you can't do. If you don't want the Tarrasque to be able to be killed by normal weapons, just say it. They have terminology for that- it's called damage resistance, and it's what we're going to use. Something this huge isn't hard to hit, it's hard to hurt. We'll give it damage resistance 20/magic weapons, so that either you're bringing magic weapons or you aren't hurting it, and lower its armor class to 20 (or 0, if you're using descending scale).
We'll keep its regeneration- 40 points now means something, since it doesn't have 800 hit points any more. 40 health out of 200 every round is seriously meaningful, and it puts it out of the reach of all but the most hard-hitting. It's still almost healing faster than you can hurt it. On a similar note, I'm dropping all the custom resistances and just giving it awesome saving throws. Now it can be poisoned... if it rolls higher than a 3. And then, what's it supposed to do, again?
I'm going to ignore the section on feats, because I don't remember what they mean but they're probably almost entirely irrelevant. I guarantee you they can be reworded as special abilities, were any of them to be interesting enough to warrant it.
And that's almost it. I'm just going to add a bit about how the Tarrasque's attacks all have the ability to either also knock you around, or stick you in his mouth and devour you instantly.
What you're left with is a creature like this:
Hit Dice: 24d10 (240 avg. health)
Armor Class: 0
Damage: (4d6+11)x6 or 24d6+66
Hoard Class: N/A
An enormous, bloodthirsty beast nearly forty feet high and seventy five feet long. Its stocky body is covered with a thick, grey hide, and its back is armored by an impossibly tough, thick carapace. It arms reach almost to the ground, and end in four thick talons. Its head is dominated by a massive mouth, two forward-facing horns, and beady, red eyes. Its roar is loud enough to deafen those nearby, and its growls can be heard for miles.
The Terrasque is immune to magic, and regenerates 40 hit points every turn. It reduces any damage dealt to it by non-magic weapons by 20 points. Every time the Terrasque attacks or moves, those nearby must make a saving throw (you be the judge) or be knocked several feet away from the Terrasque.
Those slain by the Terrasque are immediately devoured, providing 20 additional points of regeneration that round.
And there, look. An enormous, difficult to kill monster that has the same flavor as the original Terrasque (impossible to kill with weapons, impossible to kill with magic, rampages around and wrecks stuff) without being absurdly powerful or unstoppable.