What's proper English for 'experimentee'? [closed]

Subject (or research subject) may be the most common one. Guinea pig for unwitting subjects, or subjects unethically treated.

In psychological experiments a confederate is a study assistant that interacts with the subject but whom the subject believes is not involved in any such capacity. Typically subjects are led to believe that confederates are also subjects.

The thing you conduct your experiment on is its subject.

Human subjects

"The United States Department of Health and Human Services (DHHS) defines a human research subject as a living individual about whom a research investigator (whether a professional or a student) obtains data through 1) intervention or interaction with the individual, or 2) identifiable private information (32 CFR 219.102.f). (Lim,1990)"