for me it's the first time my boy (my profile pic) let me ride him. he was 10 years old when i got him and had pretty much busted hips so he wouldn't hump. he did love getting hand/blowjobs or 69s laying on our sides and asked for it fairly frequently for his age. then to my surprise he wouldn't even try to hump my girl when she went into heat for the first time since i got him. he just sniffed and licked her pussy for a bit and went to ask me "for a hand"
one day i thought that maybe we can try something different, at that point i only ever had a dog up my butt only once. i laid him on his back on the bed. it was easy, on the back getting belly rubs (which usually ended with me sucking him) was his favorite position. this time instead of sucking i just stroked him for a bit and then sort of sat over him, my knees where on his sides, his hind legs were spread and my butthole was directly above his sheath. with my hand i got his penis out, holding it just behind where the knot would form. i pushed it slowly inside my butt and he helped with a single kinda awkward upward thrust. it started swelling very quickly, i put my hand away and his penis didn't come out, we were tied. he started to pant heavily, his eyes were glassy and his penis started pulsing around my anus. it felt amazing. i made sure to keep some pressure on his knot by pulling my butt up once a while, he liked me pulling on his knot when we did anything. i don't even remember how long we lasted or when i came, i only knew that before we were finished my girl came and licked my cum off my belly/penis.
ever since he passed, the image of me sitting there on top of him, looking directly in his glassy eyes and his happy face with tongue out to the side is what i get off to every day. god, i miss him so much