The phrase "he dismounted" is grammatically correct and can be used in written English.
Example: After a long day of riding, he dismounted from his horse and stretched his weary legs.
He dismounted to yell at me.
He dismounted and pulled off his helmet.
And when the Ranger disappeared, he dismounted his horse and set about building a fire.
He dismounted and requested permission to catch behind the bat, "Informing us there was nothing he enjoyed so much".
He dismounted and gave the reins of his pony to Band, who clambered clumsily into the saddle.
Next to the yellow Datsun, he held her hand tight while she climbed down, and then he dismounted, too.
Returning to Pemberley, unshaven, with the hot horse between his thighs, he dismounts and impetuously plunges into a pond.
"I'm never exhausted; I'm a crazy m'fucker," he says, in a confident torrent, as he dismounts with a leap.
He dismounts and lowers the 85-pound Segway gently in front of him on the stairs, bouncing it down one step at a time.
Once parked, he dismounts with a calculated sweep, as if slipping from the saddle of one of his favoured quarter-horses.
When he finds the lions, he dismounts and, armed only with his shepherd's cane, he walks up to them.
