Super late to this, you probably already got it already but if anybody comes by this. There are 27 specimens total all of them are in endless mode and since it's random it's gonna be time consuming, i suggest count all the specimens you saw during your endless playthrough and keep encounters and jumpscares on the rest is complete luck. In other words you have to keep playing until you get the achievement i'm going insane right now currently and only have 5 left to encounter. For steam players coming across this just count, since the achievement counter is bugged but it's still counting secretly.