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At the risk of being a party pooper, I submitted the two images to Grok and asked if the object in the red circle is a UFO (Grok is a party pooper);-).

Here's Grok's response:

The small dark object circled in the second image (a tiny triangular or arrowhead-like shape perched on the distant ridge/horizon) is not a UFO, spacecraft, drone, or anything artificial/extraterrestrial.
It's almost certainly a small rock or boulder sitting on the crest of that far-off ridge line. Here's why this is the straightforward explanation:
The Navcam (navigation camera) on Curiosity is a relatively low-resolution black-and-white camera (1024×1024 pixels) designed primarily for hazard avoidance and drive planning, not for fine detail at long distances. Objects on the horizon several kilometers away get compressed into just a handful of pixels.
Mars' landscape is full of rocky outcrops, ridges, and scattered boulders. Small rocks perched on elevated terrain like this are extremely common in Gale Crater, where Curiosity has been driving for years. The shape can look angular or pointed simply due to pixelation, lighting angle (harsh shadows in the thin atmosphere), and the low resolution—much like how distant mountains on Earth can appear to have "unnatural" shapes in low-res photos.
The object appears stationary and in exactly the same position relative to the landscape in the original raw NASA image. There's no motion blur, no shadow inconsistency suggesting it's airborne, and no other frames from the same sequence show it moving or changing.
Similar "UFO" claims have been made repeatedly with Curiosity and other rover images (e.g., distant specks, bright spots, or dark blobs). NASA and independent image analysts have consistently shown these are mundane: distant rocks, cosmic-ray hits on the sensor, lens flares, dead/hot pixels, or image compression artifacts. No anomalous aerial object has ever held up under scrutiny in rover photos.
This particular image sparked a fresh round of online speculation (including posts calling it a "disc-shaped aerial anomaly" or comparing it to older 2014 claims), but the raw file from NASA shows nothing out of the ordinary—just another piece of Martian geology on a distant hill.
If you look at the full raw image on the official NASA site (mars.nasa.gov/msl/multimedia/raw-images/), you'll see the tiny mark is just one small dark feature among many rocks along that ridge. Zooming in only reveals more pixelation, not structure that suggests anything manufactured or flying.
Bottom line: Cool pareidolia moment (our brains love seeing familiar shapes in random patterns), but it's a rock. Mars keeps delivering intriguing geology, not visitors.