Why is the potential form of 滑る 滑れる and not 滑られる？

Question

For る-verbs we usually drop る and replace it with られる but I noticed this exception?

Answer 1

滑る｛すべる｝ is not a 'る' (ichidan) verb, but a godan verb. While it is true that all ichidan verbs end in いる or える, the reverse is not true; 入る{はいる} for example is a godan verb. 滑る conjugates as normal for godan: 滑って, 滑らない, 滑らせる etc. 滑られる is the passive form of 滑る.