Is there a command-line BitTorrent client preinstalled on OS X?

No, not pre-installed as far as I know.

Try Deluge.

It has been designed using the client server model with a daemon process that handles all the bittorrent activity. The Deluge daemon is able to run on headless machines with the user-interfaces being able to connect remotely from any platform.

^{Source: About Deluge}

Alternatively, try rTorrent, aria2, or TorrentFlux.

For simple downloading of .torrent or magnet torrents, aria2 might be your best option.

Setting up aria2:

1. Install aria2 per the official install instructions.

2. aria2 can accept .torrent links and magnet links:

   aria2c http://example.org/mylinux.torrent
   aria2c 'magnet:?xt=urn:btih:248D0A1CD08284299DE78D5C1ED359BB46717D8C'
   

Setting up Deluge console access:

1. Install Deluge per the official install instructions.

2. Run the daemon and the console process, then connect locally.

   deluged
   deluge-console
   connect 127.0.0.1:{{DELUGEDPORT}}
   

3. Add a torrent per this syntax:

   add [-p <save-location>] <torrent-file> [<torrent-file> ...]
   

4. Use --help flag for more help on commands.

Setting up Deluge remote access:

1. Install Deluge per the official install instructions.

2. Create the configuration files by starting/stopping the daemon:

   deluged
   pill deluged
   

3. Add authentication with the syntax <username>:<password>:<level>:

   echo "alice:MyC0mpL3xPass:10" >> ~/.config/deluge/auth
   

   ^{About authentication levels}

4. Allow connections beyond localhost.

   deluge-console "config -s allow_remote True"
   deluge-console "config allow_remote"
   

5. Start the daemon.

   deluged
   

Setting up rTorrent:

1. Install rTorrent per the official download & install instructions.

2. Run rtorrent to begin the interactive access.

   Use -h flag for more info on using rTorrent.