Slid down to swamp on rearing horse, encountering disaster initially (10)

'slid down' is the definition.
The answer and definition can be both to do with motion as well as being past participle verbs.
Perhaps there's an association between them I don't understand?

'to swamp on rearing horse encountering disaster initially' is the wordplay.
'swamp' becomes 'bog' (both can mean a wet or marshy area).
'on rearing' is a reversal indicator.
'horse' becomes 'nag' (term for an old horse).
'initially' says to take the initial letters.
The initial letters of 'encountering disaster' is 'ed'.
'nag' in reverse letter order is 'gan'.
'to'+'bog'+'gan'+'ed'='TOBOGGANED'

Can you help me to learn more?