Skeleton fish are a type of fish mob that was added secretly in 1.13.2, despite the changelog for that version only announcing bugfixes. Skeleton fish spawn in lava lakes in the nether. Unlike tropical fish, cod, and salmon, skeleton fish are not passive mobs, and will actively attack players when they fall into lava, though you're unlikely to notice this if burning to death.
Similarly to normal fish, skeleton fish can be caught using a bucket, though it must be full of lava rather than water. Due to the difficulty of surviving in lava, the opaqueness of its surface, and the low visibility of swimming in it, it may be difficult to find any mobs lurking within. Despite normally living in the lava, if a skeleton fish is spawned into water, it can survive as normal; The Skeleton Fish can also spawn in the Warped Reef and Phantom Nether in the Twisted Dimension, being found in the water in the former.
Similarly to the other fish, the skeleton fish cannot survive on land, and will suffocate if not submerged in a liquid. The skeleton fish also shares other fishes' measly health pool of only one and a half hearts, but it is still very quick and difficult to see through the lava, making it hard to hit.
Skeleton fish do not have a spawn egg, making them the first used mob to lack one since 1.11 added zombie villager eggs.