JavaScript is a good example. It wasn't the only web scripting language available, there was JScript and Java Applets and VBScript. JavaScript won and now it's ubiquitous.
And for an example of co-existence of competing languages, see javascript triggering flash/actionscript (e.g. for clipboard access) and vice-versa (e.g. for DOM access).
Both are somewhat dated examples, that is pre-HTML5.