Has any downgrade a Tarrasque

[Sir cryosin]

I was thinking about have a wizard to experiments. With Tarrasque DNA. And the party will have to fight the little Tarrasque.

For more info a major city in my world is Felhilm a city of spell casters that is floating in the air under the city is a large lake the size of the gulf of Mexico in size. The land was ripped up to create a prison for a Tarrasque. And the Tarrasque prison is now the lake and the floating land was turn into a city.

[Cl0001]

If you have the dm guide them look at the create your own monster section and decide what cr you want it to be, then drop it to the appropriate health and make its ac a bit lower. Then make its attacks a bit weaker and take away some of its abilities.
If I was making this, I'd drop its strength and constitution to 24 or 26 then I'd take away one attack and maybe one damage die. Drop its ac to 23 and Hp to wherever it recommends it. I'd also drop its size by a category or two. I'd keep all resistances and immunities. But I'd really need some of its other abilities. I'd keep legendary resistance and actions but make it only 1 a day/round and I'd Debate on the advantage to saving throws.

[Baptor]

I was thinking about have a wizard to experiments. With Tarrasque DNA. And the party will have to fight the little Tarrasque.

For more info a major city in my world is Felhilm a city of spell casters that is floating in the air under the city is a large lake the size of the gulf of Mexico in size. The land was ripped up to create a prison for a Tarrasque. And the Tarrasque prison is now the lake and the floating land was turn into a city.

I've not done the leg work, but I do stuff like this all the time. For example, I think it's ludicrous that a vampire is a CR 13 and that's a vampire no ifs ands or buts. I've made level 5 vampires and level 20 vampires.

The key is knowing what traits make a monster [I]that/I] monster. For example, vampires (IMHO) are known for their deadly bite, regenerative properties, shape changing into bats and mist, charming gaze, and peculiar weaknesses.

For the Tarrasque I'd say it's known for being humongus, regeneration, magic resistant, and able to swallow people whole.

Tailor hit points and so forth to the appropriate CR and throw those traits in somehow and you got yourself a Tarrasque.

[Zanthy1]

I've not done the leg work, but I do stuff like this all the time. For example, I think it's ludicrous that a vampire is a CR 13 and that's a vampire no ifs ands or buts. I've made level 5 vampires and level 20 vampires.

The key is knowing what traits make a monster [I]that/I] monster. For example, vampires (IMHO) are known for their deadly bite, regenerative properties, shape changing into bats and mist, charming gaze, and peculiar weaknesses.

For the Tarrasque I'd say it's known for being humongus, regeneration, magic resistant, and able to swallow people whole.

Tailor hit points and so forth to the appropriate CR and throw those traits in somehow and you got yourself a Tarrasque.

You're absolutely right about your vampire point, and I also agree with your tarrasque point.

Essentially if its a mini version, drop the size, hp, ac, damage/attack, and probably the legendary actions. I'd turn all immunities to resistances, because it's a knock off tarrasque. Thats at least for me, obviously some of those abilities you could keep if you deem them necessary to be a tarrasque.

[Toadkiller]

There is a good guide for this already. Dragons. They gave us a bunch (too many perhaps) each with a range of strength.

[Falcon X]

Dragon Magazine had a Petite Tarrasque. I think it was Dragon 329 if you can find access to it. If I remember correctly, it is playing off the actual mythological creature (https://en.wikipedia.org/wiki/Tarasque)