It's probably creatures whose attacks deal poison damage. There's not much to go on here, but 2 points to consider:
The DM, under Crafting and Harvesting Poison (page 258) says:
A character can instead attempt to harvest poison from a poisonous creature, such as a snake, wyvern, or carrion crawler.
It doesn't say what kind of snake, and some snakes don't have poison attacks. However, it also lists a number of poisons in its Sample Poisons section that come from this process:
- Carrion Crawler Mucus
- Purple Worm Poison
- Serpent Venom
- Wyvern Poison
Serpent Venom specifically says that it comes from a giant poisonous snake, so all of these are creatures that deal poison damage on attacks.
The second point to consider is that there aren't any rules for becoming poisoned by eating poisonous creatures, so if it's meant to mean these, it's purely up to the DM to decide what creatures are poisonous or not.