As to the horse, we can easily find some excuse for its disappearance.

Why the devil! A horse is mortal; suppose mine had had the glanders or the farcy ?" D'Artagnan did not smile.
"It vexes me greatly," continued Athos, "that you attach so much importance to these animals, for I am not yet at the end of my story." "What else have you done." "After having lost my own horse, nine against ten--see how near--I formed an idea of staking yours." "Yes; but you stopped at the idea, I hope ?" "No; for I put it in execution that very minute." "And the consequence ?" said d'Artagnan, in great anxiety.
"I threw, and I lost." "What, my horse ?" "Your horse, seven against eight; a point short--you know the proverb." "Athos, you are not in your right senses, I swear." "My dear lad, that was yesterday, when I was telling you silly stories, it was proper to tell me that, and not this morning.

I lost him then, with all his appointments and furniture." "Really, this is frightful." "Stop a minute; you don't know all yet.

I should make an excellent gambler if I were not too hot-headed; but I was hot-headed, just as if I had been drinking.

Well, I was not hot-headed then--" "Well, but what else could you play for?
You had nothing left ?" "Oh, yes, my friend; there was still that diamond left which sparkles on your finger, and which I had observed yesterday." "This diamond!" said d'Artagnan, placing his hand eagerly on his ring.
"And as I am a connoisseur in such things, having had a few of my own once, I estimated it at a thousand pistoles." "I hope," said d'Artagnan, half dead with fright, "you made no mention of my diamond ?" "On the contrary, my dear friend, this diamond became our only resource; with it I might regain our horses and their harnesses, and even money to pay our expenses on the road." "Athos, you make me tremble!" cried d'Artagnan.
"I mentioned your diamond then to my adversary, who had likewise remarked it.