Question: In my story, I want a creature to be immune to bullets, but giving it bulletproof skin seems too obvious. I want it so that you could shoot the creature a few times at its seemingly vital spots (the head, heart, etc.), but it won't die right away and can still escape or attack you. The best way to make sure it dies quickly is by draining most of its blood, which means you have to get close enough to cause a large wound with a bladed weapon.

Yes, I know you can just use a really big gun or explosives to achieve the same effect, but in this scenario, the options are limited to something relatively cheap, lightweight, and doesn't cause a lot of collateral damage. To give you some image, the creature is about as agile, as strong, and as big as a polar bear.

Preferably the reason it's immune to bullets is based on real animals, or at least something that could exist in the natural world.

Answer: 
Amorphism, if the creature has no vital organs but rather is composed of "full function" non-specialised cells that all do a bit of everything but require a certain critical mass to function as a whole. Then point damage like a bullet wound does little to no damage to the overall organism while cutting it up with a sword reduces individual sections below survival mass killing chunks of it off until the whole no longer has enough cells to grow back. Something like the [Oozes](https://www.d20pfsrd.com/bestiary/monster-listings/oozes) of Dungeons and Dragons or Calvin in [Life](https://en.wikipedia.org/wiki/Life_(2017_film)). Real world examples of amorphic life are the [Sponges](https://en.wikipedia.org/wiki/Sponge) that have mutable cell functions based on location rather than a fixed function for a given cell.
Backup Organs
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You can make it resistant to most attacks, including bullets, by having redundant vital organs. Combined with a fully shared cardiovascular system and enhanced coagulation, this seems to fit your prerequisites.

* Shooting it will hurt and damage it, but the backup organ will take over, allowing it to flee and fight another day.
* Enhanced coagulation makes sure normal wounds not targeted to drain the blood supply will not result in too much blood loss.
* A fully shared cardiovascular system allows major blood drains to be fatal.

Examples of this exist in nature where certain animals have multiple copies of the same organ. Taking one out vastly reduces their quality of life, but still allows them to survive (Octopi and their three hearts, most insects and their compound eyes etc.). Evolution is as such not a problem, your species would just take existing features and take them to the next level.