Why did Frans Hals paint monsters in his friend's portrait?

From the National Gallery in London:

"Curator Bart Cornelis explains the meaning behind a hidden monster and skull in this 17th-century portrait by Frans Hals, and how they helped to identify the sitter as Isaac Massa.

He's accompanied by one of our restorers, Paul Ackroyd, and Larry Keith, Head of Conservation and Keeper, who help reveal this painting's secrets.

🎨  Find out more about the artist Frans Hals: https://www.nationalgallery.org.uk/artists/frans-hals

🎨 Book tickets now for 'The Credit Suisse Exhibition: Frans Hals'

Open 30 September 2023 – 21 January 2024

https://www.nationalgallery.org.uk/exhibitions/the-credit-suisse-exhibition-frans-hals

Hals was one of the most sought-after painters of his generation. A gifted artist whose deft brushwork was unparalleled, he built his reputation on a new style of portrait – highly unusual in his time – that showed relaxed, lively sitters, often smiling, and even laughing.

This exhibition, the first major retrospective of Hals in more than thirty years, means a new generation can discover why he deserves his place as one of the greatest painters in Western art."