The Tarrasque probably has a very small brain, at least in proportion to the rest of its body. Much like a t-rex;
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You have to balance proportionality of the brain with the rest of its body. Consider a blue whale;
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The whale has a much larger brain than a human, and blue whales are quite intelligent for mammals. However, most people would admit that humans are smarter than whales.
If the giant owl maintains the same proportional size of its brain to that of the rest of its body, it shouldn't attain intelligence exponentially higher than a normal owl. So yes, there must be some element of magic involved with how it can speak its own language and understand several others.