Here's a thought. Can thestrals be just as fast as pegasi? What would their wing power be? Is it possible for them to have an average wing power like any other pegasi? How fast can a thestral fly?

What are your thoughts?

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I'm not an expert on wings, but you can try looking up online and doing a comparison between bats and birds such as "fastest bat" and "fastest bird." The latter may be tricky since I recall the Peregrine falcon able to reach 320 km/h by diving down, not horizontal. By the way, there are two different categories to flight speed according to the Wikipedia: horizontal and diving. I assume ascending could also be one category to consider as well.

Anyway looking up online, the Brazilian free-tailed bat was recently dubbed as the fastest horizontal flier, able to accelerate up to 100 mph.

That said, remember that Rainbow Dash can break the sound barrier, so there are some iffiness when trying to apply real world records to fiction. I'd say try to adapt that knowledge. Also consider the weight and size of the individual. Typically the smaller and lighter are faster than the bulkier.

I have my own Mythos for Thestral/bat Ponies and (in my mythos) Most Thestral are built for stealth, hiding within the shadows, etc. They are quick on hoof or in the air. Some specialize, and those that specialize in flight are at least twice as fast as your average Pegasus. Otherwise they are about on par or a tad bit slower.

And on a finishing note; No Thestral would ever be fast enough to cause a Sonic (Rain)Boom

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My headcanon is that thestrals have solid bones and greater muscle mass than pegasi which gives them a higher carrying capacity, impact tolerence, and range, but at the cost of speed and maneuverability.

It's hard to say, but given what I know about bird wings and bat wings, I would lean towards batponies being more maneuverable, ounce for ounce, while pegasi are more variable in power with some geared towards long-term gliding, while others geared towards high speed. Different wing shapes between those two.

Wow. Thanks for the input.

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In my headcanon pegasi/thestral speed depends more on magical power of individual pony, then their wings.
But on average, thestrals are usually faster than the average pegasi, and much more maneuverable. Just because their average level of inner magic approximately equal to that of equestrian unicorns.