In the series, multiple animals were able to conjure and use their own stands.
Some of them include:
How were the animals able to get their stands?
Were they all pierced by the arrow at some point?
Or did they get them some other way? Is there extra backstory that was explained somewhere else that I missed? (I've only watched the anime, I have not read the books).
Approximately 50,000 years ago, a meteorite crash-landed on Earth around an underpopulated region of Cape YorkW, GreenlandW. In 1978, a mineral excavation team uncovered an unknown virus within the rocks of the surrounding area and determined that it had been lying dormant inside the meteorite that hit Earth long ago. Despite extensive research, government doctors were unable to determine the alien virus' purpose outside the total elimination of human life. Akin to the theory of evolution, it was discovered that certain people possessed a quality that would enable them to survive when exposed to the virus, additionally granting them supernatural abilities. ~JJBA Wiki
Stands are the result of the extraterrestrial virus and the host's immunoresponse to it, which explains why Holly Kujo's stand was killing her and which makes it entirely possible that animals such as Iggy, Pet Shop, Forever, and Bug-Eaten in Part IV contracted the virus into their bloodstreams through various means.
On the surface, rogue stands such as Silver Chariot Requiem and Notorious B.I.G. in Part V, Stray Cat in Part IV and Anubis in part III might initially seem to present a glaring hole in this theory, but the fact of the matter is that viruses are microscopic life forms and the one in question is of alien origin, which means its interaction with both organic and inorganic matter after the incubation period is bound to have unpredictable results and allows for a wide range of creative freedom on Araki's part.
To sum up it up nicely, "It's an alien virus, I ain't gotta explain S#!t." - Hirohiko Araki
It seems that Stands are related to DNA. People can be born with them, like Polnareff and Kakyoin, but the Stand Arrow also gives you Stands so it must've changed your DNA code to "unlock" your Stand. The main reason I think it's DNA-related is that Giorno cloned Coco Jumbo (the turtle in Part 5) using some samples of the original, and the new turtle had the same Stand. The animals were probably born with their Stands, but the bird and the orangutan were probably Dio/Enya testing the arrow.
After reading another person's answer I remembered that the reason they stand arrows gives people stands is because of an alien virus that lives on/in the metal, this lines up with the rest of my theory because the way viruses work (I may be wrong, this is all from what I remember from middle school) is by sending instruction through RNA to a cell which then is processed and applied. The instruction could easily be something like "change DNA to unlock stand ability" but you could also be born with your stand already unlocked without needing an alien virus to unlock it for you.
