Is reptiles a common noun

Yes, the noun 'reptile' is a commonnoun, a general Word for a type of animal.

A proper noun is the name of a specific person, place, or thing; for example:

• Reptile Gardens (animal park), Rapid City, SD
• The Australian Reptile Centre, Nicholls ACT, Australia
• The Reptile Hotel (pet supplies), Birtley, UK