harry potter - Could a muggle see a thestral?

As we learn in the books:

“The only people who can see thestrals,” [Luna] said, “are people who have seen death.”

— Order of the Phoenix, chapter 21 (The Eye of the Snake)

Muggles see, and are touched by, death just as often as wizards would be. Now, the first step to seeing a thestral is being near one. But during the course of the Harry Potter books thestrals were flying over London several times.

In fact, there are two thestrals walking around Privet Drive prior to the Battle of the Seven Potters. Fleur Delacour with Bill Weasley, and Hermione Granger with Kingsley Shacklebolt both left on thestrals and they were all inside for several minutes while the steed were out front. Given that Little Whinging isn't a wizard enclave it is certainly possible someone saw them.

So given that, is it possible for a muggle to see a thestral if they have “seen death”?

Answer

I would say yes.

Three pieces of supporting evidence:

1. 
   

   Using Thestrals for transport is a serious breach of the Statute of Secrecy.

   
   

   From the first WOMBAT (transcript on HP Lexicon), if we assume that it isn’t a trick question, then we learn that Thestrals can’t be used for transport:

   
   

   
   

   Which of the following unorthodox means of transportation is considered the most serious breach of the International Statue [sic] of Secrecy?

   
   

   a. Thestrals
   b. Abraxan horse-drawn giant carriage
   c. Hippogriff
   d. Flying Muggle vehicle (eg, car, motorcycle)
   
   e. Underwater ship

   
   

   
   

   Using Thestrals to fly is only a risk if somebody can actually see the Thestral or its rider. Since this is being held up as a potentially serious breach of secrecy, I think it means that Muggles would have some chance of seeing the Thestral.

   
   

   (As you point out, Harry et al used Thestrals to reach the Ministry in Order, then they were used again to escort Harry in Hallows. So why weren’t they arrested for the latter?)

   
   


2. 
   

   It sounds like their owners are required to hide them in Muggle-populated areas.

   
   

   Thestrals are a breed of winged horse, whose entry in Fantastic Beasts reads:

   
   

   
   

   Winged horses exist worldwide. There are many different breeds, including […] the rare Thestral (black, possessed of the power of invisibility and considered unlucky by many wizards). As with the Hippogriff, the owner of a winged horse is required to perform a Disillusionment Charm upon it at regular intervals.

   
   
   

   
   

   If Thestrals were truly invisible to Muggles, then there would be no need to cast Disillusionment Charms upon them. Since no exception is mentioned for Thestrals, we may assume that any Muggle who has seen death can also see Thestrals.

   
   


3. 
   

   J.K. Rowling can see Thestrals.

   
   

   She was asked this in an interview after the publication of Order of the Phoenix:

   
   

   
   

   In the fifth book, Harry can see the Thestrals. Can you?

   
   

   Yes, I can, definitely.

   
   

   — J.K. Rowling at the Edinburgh Book Festival (August 2004)

   
   
   

   
   

   Since J.K. Rowling is a Muggle^{[citation-needed]} and does not mention any special case for Muggles, or indicate that one exists, we must assume that Muggles really can see Thestrals.