I was just thinking about the belief among Greeks that Jews worshiped the statue of an ass or an ass's head. I wonder if there was some truth to this. For pagans the idea of worshiping an ass wasn't exactly scandalous. Look at Balaam's faithful ass and the idea of a Jewish king entering Jerusalem on an ass. If you want to slander Judaism, why not claim that their temple contained the statue of an animal with more negative meaning such as a vulture?
So that makes me wonder if there was some sort of ass statue. There is also mention of a golden vine, and that sounds somewhat like Aaron's branch. Of course there is also the idea of the vine of David and the expected Messiah. IDK
Here is the article with more details:
ASS-WORSHIP - JewishEncyclopedia.com
To be fair, by the time of the return from exile, Judaism was strongly aniconic. This can even be seen in other narratives, like the Deuteronomist or Yahwist. The decalogue and golden calf is Yahwist, and aniconism is strong in the Deuteronomist history's condemnations in Joshua. The northern prophets that inform the Elohist are also strongly so. Hence when the Temple was rebuilt, the chances of a statue representing YHWH being placed therein, are almost non-existent.
Likewise, when Antiochus IV desecrated it by attempting syncreticism, he chose Zeus - and Hadrian chose Capitoline Jupiter. If Antiochus had entered the Holy of Holies and found an Ass, likely he would have chosen Dionysius as the Interpraetio Graecorum equivalent, not Zeus. Dionysius as a sort of supreme god was fairly common in Ionia, and had an association with the Ass. This is seen even further by Selinus riding on one. Furthermore, other narratives report the Holy of Holies empty when both Pompey and Antiochus defiled it. So I don't think history supports the Ass statuary being present, and it is much more likely it is just a hellenistic insult.
Why an ass was chosen is simple. Judaism has an appreciation of the ass, being a hardy animal that can survive in tough conditions. As mentioned, we have Balaam, but we also have stories of donkeys finding springs in the desert in Exodus, and even King Saul being chosen when he looked for donkeys. Prophets riding donkeys were a recurring motief, hence Jesus entered on one or the High Priest usually travelled thereby. So if you are going to ascribe an animal to YHWH with a chance of being taken seriously, an Ass would be a good bet. It also allows an association with Set or Typhon to be made, which strengthened the negative connotation of the association, if your goal is an anti-Jewish slur.
The question could perhaps be asked of the First Temple though, as the Jewish temple at Elephantine seems to have had a representation of YHWH. We also know the ethnically related Edomites and more distantly related Moabites both revered donkeys as well. In addition, the Set animal may be an Ass, and Set was a storm god. Early YHWH narrative suggests a storm god, so a syncreticism with Set in the Bronze Age, before the nasty decline in his reputation due to the rise of the Osiris legends, could be argued.
However, an equal if not better argument for a Bull or Calf could be made, based on the Golden Calf narratives of Sinai, Bethel and Dan.
Regardless if the Israelites had conceived YHWH in a bull or ass form, by the time of the writing of the Pentateuch, of all four parts in the documentary hypothesis, aniconism was established. So either there never was any such depiction, or it was already chucked out under Josiah's reforms if present, or when things like the Nehusthan were removed by Hezekiah.