Nicknames (or usernames) are unique identifiers that refer to a specific account. These usernames may be used across multiple Atelier 801 games, as well as the forum. Accounts were added to Transformice on 14 May 2010 (V0.10).
While most people refer to these as "usernames", they are officially referred to as "nicknames" by Atelier 801 staff.
Your nickname must start with a letter, can only have alphanumeric characters, and up to 3 non-consecutive underscores.
History
On 6 November 2013 (V1.120), Extinction accounts were migrated to the community platform. To avoid any nickname duplication, a "+" sign was added in front of their nicknames.
From 30 June 2015 (V1.253), nicknames could contain numbers.
From 25 July 2016, you can change your username by going to the forum, selecting "My Account", and then select "Change Name". This process costs 1,500 fraises 
. If you change your nickname to an inappropriate one, it will be reverted without being refunded. Nicknames can also now have special characters like "_" or numbers and the number of characters allowed has increased. You can change your nickname every 3 months.
On 12 March 2018 (V1.447), a # system was implemented for nicknames, allowing players to choose any nickname, even if it was taken before. Players who registered before this system (i.e. have a #0000 number) are allowed a free name change.[1]
Trivia
- Melibellule said in 2016 that spaces aren't possible in nicknames for technical reasons.[2]