The risk of actual camera damage is generally low, but it is not zero. Third-party batteries vary a lot in quality: the most common downside is lower capacity or reliability, not camera damage. Battery failures do happen, but they are relatively rare compared with the huge number of aftermarket batteries sold.

A key point is that battery problems are more likely to happen during charging. Since the EOS 550D uses a separate charger rather than charging in-camera, a battery failure is less likely to harm the camera body itself. That said, very cheap or counterfeit batteries can be poorly made, and there have been cases of batteries overheating or failing.

So: a cheap third-party battery will probably work and probably won’t damage the camera, but it carries more uncertainty than an original Canon battery. If you want the lowest risk, buy a genuine Canon battery or a reputable third-party one rather than the cheapest option available.