I haven't heard that before, do you have a source where I could read more? I know fish at least is high in omega-3s... What is it about cooking the fat that makes it bad, and does that mean raw or rendered fats are fine?

There's some evidence some animal fats may be the cause of cardiovascular disease, but it isn't great. There are a lot of confounding factors involved.

https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6053258/#sec-a.d.jtitle

(There's a lot more detail above where this links, but I linked to the end.)