Great question. We’ve asked the rental car company we work with about this before and got a somewhat ambiguous answer. They do have car seats (as they are legally required), but they carry many different kinds/brands. It just depends on what they have at the office you rent from at the time. If it helps at all, they sent us the picture below and said that they are all similar to that one.
Honestly, if you’re uneasy at all, we recommend just bringing your own so that you don’t have the uncertainty.
And if you’re still looking for a rental company to work with, you might want to check out the discount we get through Adobe. It’s between 10-25% off: https://www.twoweeksincostarica.com/costa-rica-rental-car-discount/.