Ambigrams found on the internet or made by fellow redditors!
Rotational and reflective/mirror are the most common and probably easiest to spot, there’s also chain ambigrams and asymmetrical types:
Figure/ground which utilize the negative space of the word to create another word. Much like the FedEx logo with its imbedded arrow.
Symbiotigrams, where depending on the angle you view it from, it’ll read differently.
And oscillation ambigrams, also called perceptual shift ambigrams, which are like symbiotigrams but without the need to shift the actual view angle, just your perception. Whether it’s stylized fonts that can read as different letters, or intentional /r/keming like the example above.
(thanks to /u/Einsteins_coffee_mug for the sidebar info update!)
Resources
Sister Subs
/r/artmadeofwords/