Mandatory not-Euler's :

https://en.wikipedia.org/wiki/List_of_things_named_after_Leo...

My favorite is the "Lemma that is not Burnside's". Also known as the orbit-counting theorem, the Pólya-Burnside lemma, the Cauchy-Frobenius lemma, and of course Burnside's lemma.

And then we have Grothendieck's prime (57), just to keep life interesting.

Or when a smart jerk discovered a thing, and then discovered another thing based on the first thing: https://en.wikipedia.org/wiki/Ramond%E2%80%93Ramond_field

Damn you Markov!