So I was just browsing for some pets to purchase, and I noticed that some were considered infertile . I tried looking for answers on my own, but I only saw a vague mention to an item causing it (I initially thought it was due to inbreeding or something; apparently I was wrong.)

What exactly causes an interfile pet, and why would someone wish to do so? I assume it's to make it so that others can't freely breed out a project and lessen its worth? And I also assume it's irreversible?

Inferting can be done with an Exceptional Infertility Kit or a Philter of Infertility, both available from the diamond shop. (Or by temporarily selling the pet to someone who has a kit to infert - some people make services to do this for free for other breeders.) It has nothing to do with inbreeding, and yes, it is irreversible.

The main reason is to protect the hard work people put into projects. It's especially done with max or super stat projects, since so much time and often money is put into getting stats up. Allowing people to buy fertile and breed freely would utterly wreck the stats breeding economy.

But it is also used for non-stats projects. It prevents people from buying a male and a female of the same project, breeding them, and selling offspring for a lower price than the actual owner of the project, to profit without doing any of the work. This is a lot less likely to happen (and less disastrous) than with max stats, but it still eliminates the possibility and protects the breeder's hard work.

Ah, okay, that's what I thought; thank you for the clarification :)