Eftersom denna fråga och dess svar är lite gamla, vill jag lägga till en kanoninformation från nytt material som presenteras på 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."
Om en thestral är möjlig och en häst är gemensam, bör samma enhörningshorn inte vara ett problem eftersom av storlek och om både Dragons och utdöda djur är generellt möjliga, en T-Rex borde heller inte vara utesluten.
Observera dock att enligt Spangles, att välja är din Patronus sällsynt, och eventuellt ett tecken på besatthet eller excentricitet, eller ett tecken på att de "kanske inte kan dölja sin väsentligt själv i det gemensamma livet, som faktiskt kan parade tendenser som andra kanske föredrar att dölja. "