Are malware programs that hide their true nature, and reveal their designed behavior only when activated

Trojan horses

Explanation:

At first, a Trojan horse might appear harmless and legitimate but end up being as destructive as any other malware out there. When a user maliciously downloads a program that claims to protect his computer against viruses but instead introduces them is an example of a Trojan horse. Before it is introduced into the system, the Trojan horse tricks the user into executing the malware. Once it is loaded, this malware will perform malicious actions that it was programmed to perform.