Since this question and its answers are a bit old, I want to add some canon info from new material unveiled on Pottermore.
"No reliable system for predicting the form of an individual's Patronus has ever been found, although the great eighteenth-century researcher of Charms, Professor Catullus Spangle, set forth certain principles that are widely accepted as true.
The Patronus, asserted Spangle, represents that which is hidden, unknown but necessary within the personality... []... Here, says Spangle, is the explanation for the appearance of Patronuses in forms that their casters might not expect, for which they have never felt a particular affinity, or (in rare cases) [a form that the caster may not] even recognise....
It is usual, but not inevitable, for a Patronus to take the form of an animal commonly found in the caster's native country. Given their long affinity with humans it is perhaps unsurprising that among the most common Patronuses (although it must be remembered that any corporeal Patronus is highly unusual) are dogs, cats and horses. ...
Extinct Patronuses are very rare but not unknown. Strangely, given their long connection with wizardkind, owl Patronuses are unusual. Most uncommon of all possibly Patronuses are magical creatures such as dragons, Thestrals and phoenixes. ... While a rare and magical Patronus undoubtedly reflects an unusual personality, it does not follow that it is more powerful, or will enjoy greater success at defending its caster."
If a thestral is possible and a horse is common, the similarly size unicorn should not be an issue because of size, and if both Dragons and extinct animals in general are possible, a T-Rex should not be out of the question either.
Note, however, that according to Spangles, being able to choose your Patronus is rare, and possibly a sign of obsession or eccentricity, or a sign that they "may not be able to hide their essential self in common life, who may, indeed, parade tendencies that others might prefer to conceal."
