It is not only tolerable but intentional to show fish and whale, as they might be occluded by by trees too, right?

We can say intentional, I guess.

At least it would be consistent for all kinds of animals.
It's a bit ugly visually, the whale in particular, but good for gameplay and it probably doesn't occur that often ingame that it would be too annoying

Certainly good for gameplay, if people do not like it visually they could just disable silhouettes imo

Why doesn't peacocks, crocodiles, asian elephants, tigers, wolves and zebras have silhouettes. To me this sequence seems rather random, but there gotta be something why they don't show.

Hawks and other decoratives are indeed good to avoid here (they already have a tag for that)

In D965#42302, @bb wrote:

Why doesn't peacocks, crocodiles, asian elephants, tigers, wolves and zebras have silhouettes. To me this sequence seems rather random, but there gotta be something why they don't show.

That's bizarre. Looks like this is the problem:

~/0ad/binaries/data/mods/public/art/actors/fauna$ grep basic_trans * -R
crocodile.xml: <material>basic_trans_parallax_spec.xml</material>
elephant_asian.xml: <material>basic_trans_parallax_spec.xml</material>
hawk.xml: <material>basic_trans.xml</material>
peacock.xml: <material>basic_trans.xml</material>
tiger.xml: <material>basic_trans.xml</material>
wolf.xml: <material>basic_trans.xml</material>
zebra.xml: <material>basic_trans.xml</material>
zebu.xml: <material>basic_trans.xml</material>

Hmm don't delete materials :)
Just because it's optional doesn't mean we should have actors who don't use them :) (We do but I guess it's a consequence of the past.)
About what basic_trans* is for it's as opposed to player trans supposed to render alpha as transparent. If you remove it I don't know what will happen. Also in the case you remove parallax and spec it means the game will no longer handle bump maps and reflection which would be a loss.

In D965#42326, @Stan wrote:

Hmm don't delete materials :)
Just because it's optional doesn't mean we should have actors who don't use them :) (We do but I guess it's a consequence of the past.)
About what basic_trans* is for it's as opposed to player trans supposed to render alpha as transparent. If you remove it I don't know what will happen. Also in the case you remove parallax and spec it means the game will no longer handle bump maps and reflection which would be a loss.

I'm not deleting the materials, just the reference to them in these actors, sorry if that wasn't clear.
For example, the difference between wolf and wolf_snow is that wolf has no material and wolf_snow uses basic_trans. What difference does that make? Does it hurt anything to remove basic_trans from wolf_snow?

Basic_trans enables alpha transparency (i.e. blending) in the engine, as Stan says. I think it's used in those animals for the manes or feathers they have.

The reason for this behaviour is documented in Renderer.cpp:L1402:

 		// (This won't render transparent objects with SILHOUETTE_CASTER - will
		// we have any units that need that?)

Seems like the answer to this 2011 question is "yes" after all.
I think adding a call to CallTranspModelRenderers would fix the problem and hopefully won't be a performance mess.