I feel like there should be a rule about this, but my (admittedly failing) memory and my (admittedly cursory) searching doesn’t turn anything up. There’s a rule against traveling in a vehicle. And we have the story of Usabha (Thag 2.39) who showed up for almsround on an elephant (!). He felt ashamed at his behavior, but there’s no indication it was against the Vinaya.
It’s in the gradual training:
They avoid injuring plants and seeds. They eat in one part of the day, abstaining from eating at night and at the wrong time. They avoid dancing, singing, music, and seeing shows. They avoid beautifying and adorning themselves with garlands, perfumes, and makeup. They avoid high and luxurious beds. They avoid receiving gold and money, raw grains, raw meat, women and girls, male and female bondservants, goats and sheep, chickens and pigs, elephants, cows, horses, and mares, and fields and land. They avoid running errands and messages; buying and selling; falsifying weights, metals, or measures; bribery, fraud, cheating, and duplicity; mutilation, murder, abduction, banditry, plunder, and violence.
(Found all over the suttas, e.g. MN 27)
And in the vinaya:
“One should not grab cattle by their horns … by their ears … by their dewlaps, by their
tails. One should not mount on their backs. Whoever should mount (one): an offense of
wrong doing. One should not touch their sexual organs with lustful thoughts. Whoever
touches (one): a grave offense. One should not kill a young calf. Whoever kills (one) is
to be dealt with in accordance with the rule (Pc 61).”—Mv.V.9.3 --Ajahn Thanissaro’s translation.
(Sorry for the outdated reference to Ajahn T. SC search didn’t work and it takes too long to search manually in Ajahn Brahmali’s translation, so I had to use BMC).
Awesome thanks! Embarassed to forget the gradual training reference!