There's still not a lot of BL isekai out there, but this particular one stands out by virtue of having one of the wackiest premises I've seen.
THE RUTHLESS COMMANDER AND HIS REINCARNATED WARHORSE (Shouwaru Boukun Kishu to Nagasare Senba), based on the light novel by Sakashima with art by Nomoto Narita. First published in 2019 and first published in North America in 2023.
PLOT:
Deep in a forest, a young foal is born. Once he was a modern-day Japanese man, but now he's enjoying the freedom and simplicity of life as a horse. Then one day he encounters Lieutenant Commander Felix, a strict but handsome young soldier. Felix takes this horse (now named Aleksei) for himself, and Aleksei commits himself to becoming his loyal steed. When the two are attacked by bandits, Aleksei undergoes a strange transformation that allows the two to become closer than ever before.