bcrypt_pbkdf
The key derivation function that protects passphrase-encrypted private keys in OpenSSH's own key format.
Ted Unangst wrote it for OpenBSD, and it shipped with OpenSSH 6.5 in early 2014 alongside the new openssh-key-v1 format. Before that, keys in PEM format were protected by a single round of MD5 over the passphrase, which made guessing passphrases practically free.
The construction is PBKDF2 with HMAC swapped for a variant of bcrypt's deliberately slow Blowfish-based core. The round count is set with ssh-keygen -a. It is not memory-hard like scrypt or Argon2, but the step up from one pass of MD5 was large enough that nobody was inclined to complain.