Just in case, symbiosis is a type of interraction / relationship between animals / plants / bacterias, that is usually mutually beneficial, although sometimes one party tends to get more of the benefit, and other times it turns parasitic. For example, the clown fish hides in sea anemones, and in return, the clown fish cleans the anemone. No idea how well I managed to explain this in English, but if this is wrong or if you don't understand, don't be scared to correct me / google it yourself
Alright, I had this sort of headcanon for quite some time, and I thought it was interesting enough to share – the creatures in our world form symbiotic relationships, so how would they work in the Pokémon world? The one that made me think of this was as follows.
The Viridian forest is inhabited by mostly Bug-type Pokémon, such as Caterpies and Weedles, and, although rare and not present in all games, Flying-types (Pidgeys and Pidgeottos), and the occasional Pikachu. Presuming that Pidgey and Pidgeotto would, well, feed on bugs (and since Bug is also weak to Flying), what could the poor Caterpies possibly do to protect themselves? Team up with a Pikachu to help them out, since their Thundershock could scare off the birds. Because Pikachus are rare, I'm guessing there would be multiple Caterpies and Weedles helped by one Pikachu.
I still haven't figured out what the little bugs could do in return, and their Pokèdex entries didn't help out that much, but some mention Weedle having a good sense of smell, so it could find the tasiest fruit/berries to give to Pikachu. And if we count in Butterfree as well, since it produces honey, Butterfrees could give honey to the Pikachus as a thank-you for protecting the younglings.
So, that's that. I hope this was even a remotely interesting topic to read about, and if someone comes up with amother symbiosis headcanon, feel free to share it!