Yeti's can be found during the campaign DLC or in free roam:
Free Roam
I found 1-2 at this location: X:668.0 and Y:561.1
Story
I counted 11 total during the campaign playthrough.
1) You will encounter a yeti later on in the DLC story when you are doing the objective to disable 3 generators. You will encounter him earlier in the DLC, but only get the chance to kill him once you are on your way to disabling the 3rd and final generator.
2) I found 5 yeti during the Night defenses:
1 during the 3rd on the 2nd assault wave.
1 during the 4th on the 3rd assault wave.
3 during the 5th during the 1st and 2nd assault waves.
5) Several yetis will also appear at the end of the game when heading to the Temple Shrine to find the relic. I found 1 after gaining an oxygen mask, 1 at the entrance to the shrine, and 3 in the shrine.
Killing a yeti
Once you find a yeti, he attacks in 2 parts: 1) Charge at you and 2) Pound its fist at you (typically brute enemy type attacks). The game will also give a notification once you find your first yeti, to weaken it and then do a takedown on it. You will know when it is ready for the takedown because it will stop moving and just bend over with his hands covering his face. Once he is bending over, walk around to its back and then perform a takedown (with "RS") and then mash a random button (once I got "A" and then another I got "B") on the Yeti to finish it off and get 3,000XP. The yeti will not stay bent down for more than 5 seconds, so quickly move to its backside and initiate the takedown before he recovers.
Cumulative or not?
This achievement does seem to be cumulative but only if you get a save or checkpoint after the kills because I think I killed 6-7 before it unlocked (I died while trying to grapple and it may not have kept 1-2 of my kills) so be warned.
It seemed to be cumulative but it may need to be done with 1 life, please comment below if others can confirm this, thanks!