Why is 'sărim pe calul' wrong?
This is a scenario in which Romanian and English just behave differently. There is no logical reason for it.
You need to use the indefinite article after a preposition, so in English effectively 'we jump on horse'.
The same logic as pe masă, the preposition pe takes the indefinite form. I made the exact same mistake as yours.
After a preposition like 'pe' Romanian people don't use the definite article, unless you say anything more specific about the object. It's as if the definite article is implicit with the preposition
Why "Noi ne sărim pe cal" is wrong?