What is the metagenome like for other classes of animals?
There are multiple parts to an answer for this.

1) There is not only a microbiome in the gut. So microbiomes in general are very diverse. A fish microbiome on the scales will of course be very different to the one on a human skin. 

2) Gut microbiomes are also quite different both in taxonomic and functional content, as they are in different ecological niches.

3) There are microbiome studies for nearly every type of organism I could imagine now. Fish, insects, snakes, you name it. They differ in diversity and composition of their microbiome, but also the used methods and databases do differ, so it is hard to compare them.

4) The further away from well studied microbiomes we get, the less specific will be our assignment of taxonomy and function, because we have not seen enough comparable microbes before. So more information will be hidden in the "dark matter" (in metagenomics: reads not assigned to a taxon and/or function).

Unfortunately I don't know of any jellyfish microbiomes so far. Their body (as far as I know ) seems to be much more "permeable" for sea water (-> their environment) and thus I would think that they generally would have aquatic bacteria in whatever corresponds to their gut. The sea microbiome is said to be very diverse and far too little studied, although there are cool projects like the Global Ocean Sampling Day to try and close this huge gap as best as possible.