Cherrim forms come in two varieties, along with its pre-evolution Cherubi, too.
As with anything forms-related it's never entirely simple, so below well explain all you need to know about the two Cherrim forms and Cherubi too, including both how to get Sunshine Form Cherrim and how to get the more common Overcast Form Cherrim, too.
Cherrim forms explained: how to get Sunshine Cherrim and Overcast Cherrim in Pokémon Go
Cherubi evolves into Cherrim, and Cherrim can have two forms: Sunshine Cherrim or Overcast Cherrim.
In the mainline Pokémon games, Cherrim's form would change on the fly - if a battle had Sunny weather conditions, for instance, then a Cherrim would appear in battle in its Sunshine form. In any other situation it'd appear as standard, Overcast Cherrim.
In Pokémon Go, things are similar but not exactly the same. Pokémon don't change their forms dynamically, they're set as one form or another. In Cherrim's case, that means Sunshine Cherrim seems to only appear in sunny weather (more on that in our guide to Pokémon Go weather conditions) in the wild. Overcast Cherrim seems to be what appears in all other weather conditions when it spawns.
It also seems to be the case that evolving a Cherubi will always cause it to evolve into an Overcast form Cherrim, and not a Sunshine one - it seems, at least for now, that Sunshine form Cherrim is only available via catching it in the wild during sunny weather.
The Season of Mischief is here! Current events include the Season 9 of Go Battle League, the addition of Raid Achievements and the Misunderstood Mischief research quest. Shiny Ditto can also now be found in the wild and this tricky Pokémon is now appearing as new creatures! Meanwhile, the last major update saw a level cap increase - including the addition of XL Candy, boosts to some XP sources and the addition of Platinum Medals. Inkay and Malamar are now in Pokémon Go!
How to get Cherubi, Cherrim's pre-evolution in Pokémon Go
Cherubi is a pre-evolution of Cherrim, which would make you think it's more of a 'baby' Pokémon that's only available in Eggs.
Thankfully, that's not the case: there are some reports from players on the Silph Road subreddit confirming that Cherubi is available to catch in the wild, especially when you use special Mossy Lures, as well as reports of Cherubi in 5km Eggs.
It doesn't seem to be Lure-exclusive either - it appears to be the case that you can catch one out in the wild without a Lure - so it seems like tracking this little cheeky cherry Pokémon down is a case of good luck and persistence, although Mossy Lures and Eggs are a good way to narrow things down!