PrEformed vitamin A comes from animal sources and is ready to go...the body doesn't have to convert it. It includes retinal, retinyl, retinal, etc. Beta carotene comes from veggies, greens, etc. and is prOformed and has to be converted into vitamin A in the body as do other carotenes. PrOformed sources can't build up in the body and lead to health issues, prEformed sources can.
There is controversy as to whether any or all chameleons can convert the carotenes to prEformed or not.