TNT ("The Neopets Team") is slang for the Neopets support team.
The Neopets Team consists of all the employees who make Neopets possible. There are several notable members who tend to either interact more with the community or who end up being parts of events, such as Mr. Insane, Snarkie, and Viola. You can visit the userlookup of their official account here, which, upon entering, looks like this:
This account is used for the sole purpose of sending messages and modding. It's not a real staff member but a bot.
When this account posts on the neoboards, their active pet is a darigan poogle named rackatackatackata.
The TNT avatar is used by the official account and is not accessable for normal players.
Specific staff members also have their own accounts and will sometimes interact with the community. Snarkie tends to be one of the only staffers who does this and thus carries a bit of a larger fanbase than other employees.
You can view the official "About Us" page here.