What is the equivalent of the RSPCA in Belgium?

Try Gaia - https://www.gaia.be/nl
or Animal Rights Belgium - https://www.animalrights.be/

There is no national system equivalent to the RSPCA which has a quasi governmental role and is recognized in law, with RSPCA inspectors having legal and enforceable rights.

There are a number of organisations like Gaia and Animal RIghts mentioned above, but also others such as the Croix Bleue https://www.croixbleue.be

However, typically your first point of call in Belgium would be your local commune or the local police. For example, if you live in Brussels, the commune has a dedicated page for how to report suffering of animals. You can report in-line, or in an emergency, it advises you to contact the police.

https://environnement.brussels/citoyen/services-et-demandes/plainte-sign...

Wallonie has a similar page:
https://www.wallonie.be/fr/demarches/porter-plainte-en-cas-de-negligence...

I assume Flanders has similar but I am not a Dutch speaker.