As much as I'd love to see the stats for this thing...unfortunately same-type bonuses don't stack.
Both Wish and a Tome of Understanding provide an Inherent Bonus to your Int. And the Wish spell even has this quotation as part of the spell description
Note: An inherent bonus may not exceed +5 for a single ability score, and inherent bonuses to a particular ability score do not stack, so only the best one applies.
So, If you both use Wish and a Tome of Understanding...your Tarrasque is still only going to have a +5 to its Int.
Now...that's not to say you can't still make an Intelligent Tarrasque. An Intelligent spellcasting Tarrasque could feasibly have been the result of someone trying to abuse the Wish spell.
You may try to use a wish to produce greater effects than these, but doing so is dangerous. (The wish may pervert your intent into a literal but undesirable fulfillment or only a partial fulfillment, at the GM's discretion.)
So, if you had a sufficiently reckless NPC at some point in the past who made a very poorly phrased Wish, and the spell either turned him into a Tarrasque, or granted the existing one boosted Int and Wizard levels, or just created one out of nothing.