Hi there. 

Biology is necessary to write the NEET examination. If you had Biology in Class XII then you can surely write the NEET examination immediately or after completing Engineering.

To be eligible to write NEET one has to be between the age range of 17 - 25 years of age with at least 50% ( for UR category) aggregate marks in class  XII boards with Physics, Chemistry, and Biology. 

Click the link below for more relevant information: 

Https://medicine.careers360.com/articles/neet-eligibility-criteria

Hope this helps.