All of Tahm Kench's "Rivals" are Yordles

Scroll down to the bottom of the page and see for yourself.

Also, Fizz is included, perhaps concluding that Fizz is actually a form of Yordle too? The only one missing is Veigar, though Tahm and Veigar have some twisted desires in common, so maybe that's why.

Finally, is Tahm a Darkin? His only "friend" is Aatrox, and Aatrox also feeds off human suffering, albeit in the form of death in war rather than Tahm's appetite for human misery...