Looking for all "boots" tags, there are a lot of them! http://e621.net/tag?name=*boots*&type=&order=name&commit=Search

At present there is exactly 1 implication and 0 aliases involving the topic of boots.
http://e621.net/tag_implication?approved=all&order=tag&query=*boot*&user=
Implication: thigh_high_boots -> boots

Previous threads on this topic: forum #26515

Types of Boots

There are three major types of boots that are easily identifiable from their size:
Standard Boots which are ankle length. Commonly seen in work environments as steel toed boots or in recreation as hiking boots. They can go a portion of the way up the calf, but never more than half-way.
Knee-High Boots are predominantly for fashion but also seen as things like galoshes. I could say that word pretty much all day. Galoshes galoshes galoshes. These boots end just below the knee.
Thigh-High Boots are seen in fetish wear and pretty much nothing else. These boots generally end more than half-way up the thigh. Importantly, they always cover the knee.

Implications

Standard Boots would simply be tagged "Boots" and the wiki can be used to clarify further. An alias could be set up if deemed necessary.
Knee-High_Boots implies -> Boots
Thigh-High_Boots implies -> Boots

Aliases

The existing tag of thigh_high_boots ought to be changed to Thigh-High Boots which is the commonly accepted way to write it. Googling the two shows ~4 million more hits for the spelling with a dash in it.
Similarly, Google shows a strong preference for Knee-High_Boots over other wordings of it.

Knee_High_Boots should be aliased to -> Knee-High_Boots
Thigh_High_Boots should be aliased to -> Thigh-High_Boots
thigh_boots should be aliased to -> Thigh-High_Boots
knee_boots should be aliased to -> Knee-High_Boots

Additional Discussion

cowboy_boots are distinctive an iconic enough to warrant their own tag I would say.
high_heeled_boots? Tag them as the appropriate boot length + high_heels?
ballet_boots?
combat_boots?