There's this card type in Yu-Gi-Oh that I can't quite put my finger on. It has to do with nature
It's not completely clear yet what year was this. From your post one can suppose you mean around 2015 (you said 6th grade and you imply you are now on 10th, so 4 years have passed).

--If clarification is made I'll gladly update my answer--

Assuming the cards are around 2015, and that are nature-ish themed, and some of them ride beasts, I think you are talking about the Ritual Beasts Archetype. 

They were introduced mostly on the Secrets of Eternity booster expansion, and on the The Secret Forces expansion as well, which were launched on TCG January/February 2015, respectively.

Here some images of some cards of that Archetype to depict the theme they display (nature-ish, riding beasts, anime/cutesy, etc.):

Ritual Beast Tamer Lara:



Ritual Beast Ulti-Gaiapelio: