Charles Spencer's youngest child, daughter Charlotte Diana, 11, appeared in a previously unseen photo shared to social media at the weekend.
The youngster wore jodhpurs and a jacket with riding boots and hat and beamed as she held the lead of a small brown pony covered in a purple blanket, with a yellow lab sitting by her feet.
Her mum, Countess (Karen) Spencer, who posted the image to Instagram, captioned it with a poignant message.
It read: "In the last month, we've have had to put two of our beloved pets down here at Althorp. Roo, Charlotte’s first competition pony and our beloved lab Otis. Otis was definitely my dog of a lifetime. I can't believe he's gone [broken heart emoji].
"Here they are with Charlotte in happier times in front of the Falconry at the back of the park."
The tragic news comes weeks after Charles and Karen announced their divorce, and the Earl also paid his own moving tribute to Otis on social media, where he wrote: "Everyone at Althorp loved Otis - several people here have talked today of his having been a 'total legend'. The place won't be the same, without his wise, sweet, intelligent presence. He was extremely special."
Earl and Countess Spencer's split
In an interview with the Mail on Sunday in early June, Charles revealed: "I just want to devote myself to all my children, and to my grandchildren, and I wish Karen every happiness in the future."
He was later photographed enjoying a theatre outing with one of his podcast co-hosts, Dr Cat Jarman. The father of seven, 60, married Karen, 52, in 2011 after the couple were initially set up on a blind date in Los Angeles the previous year.
Charles proposed in the grounds of his family estate, Althorp, which is in Northamptonshire.
Who is the Countess of Spencer, Karen Gordon?
The Canadian philanthropist is the founder of Whole Child charity and a mum of three, having welcomed daughters Emma and Kate during her previous marriage to Mark Gordon between 1997 and 2003.
She gave birth to youngest daughter Charlotte in 2012, and has been raising her at the ancestral Spencer residence since then, while playing a major part in renovating the property. Charles, meanwhile, is the 9th Earl Spencer, and the maternal uncle of Prince William and Prince Harry.
A historian, writer and broadcaster, he is also the custodian of Althorp and proudly shares pictures with followers across social media.
All about Althorp
The Grade I listed stately home boasts a sprawling estate and is located five miles northwest of Northampton, between the villages of Great Brington and Harlestone.
It's been in the Spencer family for over 500 years and is set on an incredible 13,000 acres, with the house and gardens comprising 550 of those. Charles took over the property after the death of his father John in 1992.
Princess Diana was laid to rest on an island on Oval Lake in the grounds of the house but her resting place is only accessible via boat and is off-limits to the public.