Dinosaurs. In preschool/kindergarten I wrote and drew in a journal every night before I went to bed, and when I read it now, I spelled words like "there" wrong (ther) and everything but I got all of the names of the dinosaurs right. I was absolutely obsessed with the Land Before Time too, but I didn't understand why they were called things like longnecks and threehorns instead of apatosauruses and triceratopses or why said dinosaurs existed in the same time period.
I had all of these pretty intense non fiction books and movies on dinosaurs too xD I remember in 2nd grade the librarian always told me, when I got non fiction books and chapter books, "Now go pick out something you can read on your own." And I was like, "I can read those on my own..." But I had to get one of those Step into Reading/Hooked On Phonics/Whatever-they-call-them-now little kid books since I "couldn't read my dinosaur books on my own."
Also in preschool, I wanted to be a paleontologist. There were only 2 or 3 other girls in my preschool class, and about 20 boys. But for some reason the girls thought I was the coolest thing ever for some reason and so they wanted to be paleontologists with me.
Ah, the good old days.
Then horses in middle school, and I got made fun of for that. Not sure why, but I got bullied by kids about horses.
Then playing guitar and music theory a bit later.
Then betta fish briefly.
Now dog behavior and operant conditioning.