Paras in depth
The fungus that drives the bug
The design is based on a cicada nymph infected by a fungus from the Ophiocordyceps genus and still alive. The tochukaso mentioned in the Japanese text is real: 冬虫夏草, winter worm and summer grass, a fungus that parasitizes insects and that traditional Chinese medicine has sold as a remedy for centuries. That directly explains the entry about the life-extending elixir, which is not something the writers invented but a description used to market the product.
The unsettling part is the behaviour. Real Ophiocordyceps manipulate the ants they colonize, leading them to a place where the fungus can sporulate to its advantage, and the Pokédex reproduces the mechanism when it says Paras digs to feed mushrooms that are not its own. The name does not complicate matters: パラス in Japanese and Paras in English and Spanish, shortened from “parasite” in all three cases, with the deliberate question of which of the two organisms it refers to.
When Spore stopped belonging to it
In the first and second generations, only this line could learn Spore, and that was enough to justify a place on the team. Later, competition arrived with better numbers: Breloom inherits the same move and adds much higher Attack with a Fighting typing, while Amoonguss combines it with plenty of HP and Effect Spore as well. Paras was left with the trick but without the body to make use of it.
The second blow was a rules change. Since Generation VI, Grass types have been immune to powder moves, including Spore, and Safety Goggles do the same for anyone holding them. A move that used to be infallible gained a list of exceptions that any opponent can cover with an item. Dry Skin, added as a second ability in Generation IV, patches up the Water side a little and makes Fire, already its worst weakness, even worse.
Where to find it and what to do with it
It is a regular in Mt. Moon and the Safari Zone in Kanto, appearances that have been repeated in every remake of the region, even though its habitat is Forest. Finding it in a cave fits the bug’s biology: a damp place without sunlight, exactly what a fungus needs.
There are two sensible ways to use it in a playthrough. The first is not to use it in battle at all: keep it on the team until level 29, learn Spore and devote it to putting everything you want to catch to sleep for the rest of the game. The second is to train it properly, accept that it will take heavy damage and make use of Effect Spore punishing anything that touches it directly; against a physical opponent that relies on contact, the roulette of sleep, poison or paralysis decides battles that its 285 points should not be deciding.
What you should not do is send it in against Flying or Fire types. With two quadruple weaknesses and 35 HP, there is no room for an optimistic calculation.