Is there an issue with ladies going to the zoo or looking at animals in general when they are pregnant? I've heard it may be specific to ugly animals (e.g. monkeys) so as not to scare the mother. Or is it Kabbalistic in nature? Does anyone know of a source for this halacha / custom?
The mekor is based off the Sefer Habris which explains it is not good for a pregnant woman to see strange things and then get scared which is bad for the baby. See here for answer #399 .
This discussion is brought down in the Sefer Vayehi Binsoa in perek 14 on the section discussing birchas meshaneh habriyos. In the Orchos Halacha section it notes that Rabbi Pollack heard from the Rebbe of Pshevorsk that he reasoned; he has never seen a lion, but since the mishnah says that one should be "גבור כארי" ("mighty like a lion") it indicates that it is quite fierce, and one can possible assume that a pregnant woman might become scared. And thus, this is the primary concern, not because it is a tamei (impure) animal. However, this teaching was based primarily on the premise that he had never actually seen a lion. Accordingly, if a woman regularly visits the zoo and knows she won't be scared, then there is no problem with her going, and Rabbi Yitzchok Tuvia Weiss ruled that it was ok for a pregnant woman to visit.
It also mentions the Sefer HaBris that has already been mentioned above as well as the Sefer Invei HaGefen (2nd cheileck, 12:10) that if she will be transfixed at the sight or deeply scared which will make a profound impact on the foetus, then she should refrain from visiting. However, similarly, if she is used to going and there is no worry about her being scared etc. then it is permissible. However, there is still the concern that one might gaze at it for too long a time which is also problematic according the Kav HaYashar 2:7.
After a bit more searching I saw in the Halachically Speaking series entitled The Expectant Mother, that it brings the machlokes with further sources. (I will try and look them up if they are accessible online)
Some poskim are of the opinion that a pregnant woman should avoid going to a zoo.1 However, others are lenient with this.2
1 Migdal Oz chalon tzurey 32. Opinion of Harav Tuvia Goldstein zt”l (as related by Rabbi Yisroel Dov Webster Shlita).
2 Bunei Beischa page 332, Opinion of Harav Felder Shlita, see Ashrei Yiladito page 31:footnote 25 who is lenient.
I see two mekoros; the sefer habris and the kav hayashar. I have been unsuccessful in finding any mekor in sifrei halacha and minhag (bar ilan didn't offer anything, but i'm not the greatest at searching...). I don't have access to these two great sefarim, but then again i understand that these sefarim say many things that we don't necessarily follow. I heard about this minhag from my mother and mother in law, and I would like to know if this minhag is the product of folklore so to speak, or it is actually endorsed by rabbanim.