I was just going over the list of tags, and noticed that there are both 'novel' and 'novels' tags.
Is there a way to merge these into one? 'Novels' is more popular.
Possibly related: Tags: Novel or Book?
Moderators can merge tags. I've just merged novels into novel which was prefered by 100% of the people on chat at the time.
In general, if there's a community consensus that two tags are synonyms and should be merged, a moderator will do it (and if we forget or miss it, ping one of us here on meta, or on chat). For a singular/plural pair, it's obvious that they're synonyms.
Moderators cannot mass-delete a tag. Developers can, but if there are only a few instances, they can be removed by hand. Developers can also blacklist a tag, preventing them from being ever added to a question, if necessary.