I use far more tags than would ever fit on the sidebar. This page gives you an idea of what other tags there are.
All Posts Featuring Rainbow Dash | Pony Only | Humanized Only
As a Filly | Rule 63 (Male versions) | Humanizations with People of Color Only
Some posts have more than one of these tags.
Posts tagged as evidence that Rainbow Dash is a physics defying badass
Rainbow Dash Ships:
Appledash | Dashpie | Flutterdash | Twidash | Raridash
Dashfire | Soarindash | Gildadash | All Ships (not 100% complete)
Wallpapers | Videos | Gifs | Comics | Music | Ask blogs
Rainbow Dash & Pony Cosplay (also includes tips/merchandise)
Cupcakes/Food (Pony-inspired food Mmmm!)
Colorful Hair (People with colorful or rainbow hair but not cosplaying)
All Items | Plushies | Sculptures/Figures | Customs | Toys
How to style and care for your pony toys/figures
Stuff featuring or focusing on Cutie Marks
Official Hasbro Stuff | Everything Else/Misc. | Bags
All Clothing/Stuff You Wear: Hats | Shirts | Jewelry | Scarves | Pins | Shoes
Fluttershy | Pinkie Pie | Rarity | Twilight Sparkle | Applejack
Mane Six to see the six of them together | Spike | Tank
Princess Celestia | Princess Luna | Nightmare Moon
I also use a Scootaffection tag but it mostly overlaps with the Scootaloo tag.
Fanart with nice backgrounds. Some overlap with locations/events.
Fanart completed with traditional (non-digital) media
Crossovers between MLP and other Fandoms
Scenes from the show humanized (One of my most favorite things!)
The ponies depicted as other species (not pony, not human either)
Real Life Meets Ponies
In the future I will probably have a separate page linking all tags related to specific episodes. I’ve been trying to tag posts with what episode they’re referring to or come from if they’re from a specific episode.