Are your PCs the only living creatures in your world which gain

I use some custom rules for experience. You need to train. You don't go to bed one night and wake up higher level. Instead you gain the ability to advance after sufficient training.

Literally everyone can do this. NPC, PC, animals even.

>>51451767
Only things with Int 3 or higher can gain experience, that rules out most "common" animals. At least that's the way we play it.

In my setting, snakes, lizards, and other reptiles that "level up" can become dragons, and rather than age normally they grow and "age" by killing and eating things, similar in how the PC's gain experience from killing things. Admittedly inspired from Dark Souls, but I find it lets the PC's know just how fucked they are when they see an ancient dragon, which pretty much had to commit genocide to get that big.

File: dragonkoi.jpg (46KB, 280x400px) Image search: [Google]

46KB, 280x400px

>>51451999
>>51451798
>tfw you were just a fish but you trained so hard you became a dragon

>>51451767
Sounds like you've been playing too many videogames or reading too many trashy weeaboo comics based on video games.
Experience is supposed to be an abstraction of literal experience and growth, not some magic energy you take from fallen foes or get when you complete a quest.

Good gms will have a living world that develops beyond the direct actions fo players and rivals/enemies that grow in power alongside the players are far from unheard of.

>Would a spider gain experience points just from killing and eating insects?
Assuming d&d from your post, monsters and animals generally advance through hit dice. It's not really worth tracking experience unless they're a player or a recurring enemy/rival/ally.
>Are giant and dire versions of animals just high level animals?
No but you could certainly change that for your personal game.

>>51453704
In 4e everything that enters combat has a level of some form. A bear's level would normally be around level 6, a dire bear 11, and owlbears around 8.
But the DM has tools to adjust the standard level of anything so that your Demi-god level 28 players could still have trouble with a bear if you so chose.
But a good example for the OP's question would be the animal companions that many fantasy games allow rangers, druids, or whatever their natural dudes they have, to use as allies. These (essentially) monsters raise in level with you, predicated on whatever system they are based in.