There are 2 choices for the alternative default.plymouth (providing /lib/plymouth/themes/default.plymouth).

  Selection    Path                                                           Priority   Status
------------------------------------------------------------
  0            /lib/plymouth/themes/ubuntu-logo/ubuntu-logo.plymouth           100       auto mode
  1            /lib/plymouth/themes/ubuntu-logo/ubuntu-logo-scale-2.plymouth   99        manual mode
* 2            /lib/plymouth/themes/ubuntu-logo/ubuntu-logo.plymouth           100       manual mode