Squirrel brains, cow brains, human brains ... mammal brains in general.
More interestingly it's rare - you have to eat infected brains that have a variation that side steps not one but two prion defence mechanisms that most humans have ... which suggests that almost all humans have ancestors that engaged in enough cannabalism to develop layered defences against brain transmitted diseases.
Why would we jump to cannibalism if it is merely sufficient to eat a bunch of other mammal brains (which I vaguely remember reading are a "delicacy", though I feel like I hear that about enough extremely dangerous foods that I consider it an anti-endorsement at this point)?
As a general rule of thumb, diseases varients that evolved within mammal type X proliferate the most within mammal type X populations - the varient can jump to mammal type Y but tends to be a relatively rarer occurrence and tends to go with a bit of further evolutionary variation.
FIV is not the same as HIV (Feline, Human, but they're similiar) and so to with prion diseases.
Mad Cow disease came about from feeding large quantities of ground up cow offal from abattoirs mixed up with grains to make 'tasty' feed pellets .. eventually a cow prion disease 'perfected' itself and went on to occassionally(?) affect humans - more common in cows than humans, etc.
With human cannabilism, we know about Kuru from the Fore people in PNG who, like many other groups in the highlands, practiced cannabilism as a mortuary ritual, a funeral practice to retain the essence of someone respected. Kuru was a specific human to human variation that evolved to become more common and overcome the defences against human -> human prion defence I mentioned earlier.
We have other reasons to suspect cannibalism was common elsewhere .. such as the documented European practice of medicinal cannibalism and the fashion of consumming bitumised mummies.
But, ahh, yeah - these days genetics is the thing, markers for defences against human prion diseases are a thing talked about in a group of papers post Kuru.
They're high in fat even in otherwise lean animals so would have been a totally normal thing to eat in much of the world for most of history. In some places & seasons possibly the only ready source of saturated fats, so probably a delicacy for that reason.
Prion diseases https://www.cdc.gov/prions/index.html lurk within brains and nerve stems, with cows it became a big thing when the industry started making mega tonnes of offal waste mixed with grain feed pellets .. so all cows were being feed ground up other cows including brains.
Like most other diseases species -> same species is an easier vector than species -> different species.
More interestingly it's rare - you have to eat infected brains that have a variation that side steps not one but two prion defence mechanisms that most humans have ... which suggests that almost all humans have ancestors that engaged in enough cannabalism to develop layered defences against brain transmitted diseases.