Horizon Forbidden West

POSSIBLY MISSABLE

There is 1 missable machine, the Specter, which does not spawn the open world. However, there are several main quests it appears in so it's unlikely you'd miss scanning them. Note: The final boss, Specter Prime, used to be missable but as of Patch 1.08 (March 16, 2022), it no longer requires to be scanned. Once you encounter it, you will automatically get the scan/Notebook entry. There is a possibility that the same applies to the Specters in the final mission but this is as of yet unconfirmed.

The 43 different machine types are as follows (in the order that they are in the Notebook area of the menu screen - meaning if you miss one, you know exactly what you’re missing):

Grazer
Burrower
Scrounger
Leapleasher
Scrapper
Spikesnout
Skydrifter
Plowhorn
Charger
Lancehorn
Longleg
Bristleback
Fanghorn
Redeye Watcher
Widemaw
Clawstrider
Corruptor
Bellowback
Glinthawk
Shell-Walker
Stalker
Grimhorn
Ravager
Snapmaw
Sunwing
Clamberjaw
Rollerback
Rockbreaker
Behemoth
Dreadwing
Tideripper
Shellsnapper
Stormbird
Thunderjaw
Frostclaw
Scorcher
Sliterhfang
Tremortusk
Fireclaw
Slaughterspine
Tallneck
Spectre (MISSABLE - during many main missions)
Spectre Prime

The best strategy is: unlock all the Tallnecks (location guide here), play the game as you want (making sure to scan the 3 missable machines during the relevant story missions) and then mop up the rest post-game. There’s a very good chance you’ll have the vast majority come the end of the game. so will only be missing 2 (the Corruptor and the Spectre Prime. Just make sure you scan as much as you can.

Note: The Corruptor machine is not missable. However, it doesn't spawn in the open world. If you miss its appearance in the 13th main quest, "Faro's Tomb", the only other place you can find it is in the Arena's highest difficulty set, the 2nd of the Legendary challenges.