Hi there Blindfoot, welcome to TEFc
I'll tell you how to get a mini-deer spell but keep in mind you should wait until your deer grows up to get it. Otherwise the spell will wear off on your fawn and you will loose it!

The Mini deer spell is one of the transformation spells (the ones that turn you into other forest animals, like doves, frogs, bats, or squirrels). Like most spells you need to have another deer cast it on you, as you are unable to cast spells on yourself. Have another deer sit inside a mushroom circle and fall asleep (tip: your deer will fall asleep on it's own if you don't move your cursor for a few minutes). Once they have fallen asleep, they should have access to the ability to cast a transformation spell. However, the spell they cast is random and it will likely take several tries because you get the mini spell. Once you have the mini spell, be sure you save it using the save button in the game menu!

It doesn't mean anything about a slenderdeer, as slender is a character played by a player on here with no affiliation to the game's creators.

Weather changes could be
*a celebration (i.e. Christmas, Halloween, etc.)
* A random occurrence
*An Abiogenesis Celebration

It is just another time to have fun and maybe even glimpse the Gods running around.