|He looks like this.|
Tarrasque means D&D monster who exists to be a big bucket filled with HD and multiple attacks and ridiculous AC and also you have to redraw your maps now because it destroyed all the cities. It literally feeds on destruction, death, and misery.
Tarrasque means all of that in 4e but in addition he can never truly die, he just keeps coming back after he has healed up, the looming constant threat of natural disaster as real as plague or earthquake that everybody fears will one day happen but which no amount of effort will prevent. Like Yellowstone, actually. Not an omnipresent terror, but, in a land of a tarrasque of this manner, a constant anxiety.
Alien yet familiar creature + dumptrucks of castle-destroying hit points + oh god damn it there he is again I knew it = Arcis Enumre has a Tarrasque and it's Anguirus. Knowing that also means the Tarrasque is fast, can move bipedal at a slower rate, can make himself temporarily impervious to even magical weapons, and can appear anywhere without warning.
Also knowing that, it lives on an island with about 30 creatures way more deadly than himself. Every few years, one of them will leave this island, and when they do the rest will follow. This is partly because the animals will starve to death without wreaking some occasional havoc but their home is also really far from civilization and being that huge makes you kind of lazy too. It's also a salmon-like journey to spawn, but since each creature is the last of its kind this simply makes them pissed.
I have a scroll of Summon Tarrasque in my game as well. Casting from that scroll not only summons Another Tarrasque, but the tarrasque will immediately leave to come find it and murder-mate with it, setting off the monster domino effect. Summoning a Tarrasque is a perversion of nature, a blasphemy, a crime against the world, and so the spikes on a summoned Tarrasque are the horns of unicorns. Each round a unicorn can burst from the flesh of a tarrasque, which scars over, and will chase you. The tarrasque can spawn up to 666 unicorns because why not. Summoned tarrasques are male.
When you're killed by the summoned tarrasque or its unicorns, you go before the Ur-Gods of the world, Thunder Queen, Spirit King, and Great God Good Weed, and are judged and sentenced by this Tribunal Ultimus.