Among the given animals which one isn't a real fish?

Vertebral column, fins and gills are characteristics of a true fish.

Silverfish is the common name of Lepisma which is an insect belonging to the phylum Arthropoda. It is shiny silvery-grey in colour and moves like a fish hence called silverfish.

Sea horse (Hippocampus), Silvercarp, Goldfish are examples of true fish as they fulfill the criteria of true fish.