Where did the name Thanos come from?

The name Thanos originates from the ancient Greek word Thanatos, meaning "death" or the personification of death in Greek mythology. In real-world Greek culture, Thanos is also commonly used as a shortened, everyday nickname for the traditional name Athanasios, which means "immortal".

I never met anyone named Thanos.

Valentino is a masculine given name of Italian origin, derived from the Latin name Valentinus. This Latin root comes from the adjective valens, meaning "strong," "healthy," or "vigorous."