See How to Play Offline for how to play Chess960(shuffle chess), Crazyhouse, Atomic Chess, Losers, Giveaway/Suicide, or Three Checks against the Pulsar Chess Engine with a choice of Levels.
Both Guests and Members can play variants online. Guest play is unrated and unrated seeks are hollow on Seek Graph.
To issue an online seek for a variant game go to Board's Find a Game option. This is a tabbed view with selector at top and opens to seek a game. Variant is the last option if the user doesn't want Chess so scroll it down as neeeded. On FICS the default formula excludes variants and very fast games, so have the formula option in seek a game form on off if a guest or new user on FICS.
Diamond Chess shows the major variants that are offerd on FICS. On ICC there are more choices, but the more interesting ones are toward second half of list including Chess960 a popular form of shuffle chess.
In Chess960, the Pulsar Chess Engine play supports king captures its own rook to indicate a castle move with that rook, In online play on ICC, type in Main View oo, to make a king side castle move, or ooo, to make a queen side castle move.
Users can also on FICS type set formula in console if a guest or as a member do the same to clear their default formula. Or type help formula in Main View to learn more such as how to make the user's own formula to filter seeks. Clearing the formula will also allow FICS users to see variant seeks on Seek Graph and fast games under three minutes. Seek Graph is another tab on Find a Game. ICC has no default formula.