We're introduced to an amazing creature, a living bio-ship named Gomtuu (called 'Tin Man' by Starfleet) who was observed while orbiting Beta Stromgren in TNG 3x20, 'Tin Man'. Through first contact specialist Tam Elbrun's telepathy, we learn that Gomtuu is actually a living and sentient starship. We also learn some of his backstory including the loss of his crew.
The two large space faring creatures featured in TNG premier, TNG 1x1, 'Encounter at Farpoint' don't qualify as bio-ships. Though these obviously sentient creatures can 'transform' into ships and other structures, they're not 'ships' in the strict sense.
Have there been any other bio-ships encountered in the Star Trek universe?