I can't remember what group I was in, but someone asked about converting 5e monsters to OSR/BFRPG. This is how -I- would do it. I usually work with S&W so this answer is specific to that, but all the OSR games are pretty close, so...
If you don't want to do this, that's fine! It's YOUR game, play it how you want!
- - -Only four things really matter in OSR: HD, AC, Saves, and Damage. Save(s) are HD-related, so we're down to three.
HD - Do NOT use as written. Compare Challenge and attack bonus. HD will be somewhere around there.AAC - use as written +1 (since AC 10 is unarmored in 5e, vs AAC11 in S&W). 5e has a reduced AC range compared to 3e, so it works fine.
Damage - Damage is tricky. Very roughly, add the creature's damage dice together, then divide them by either 2 or 3. Divide those between the creature's attacks. Drop modifiers for now.
Ability Scores - I keep the ability bonuses, but drop them by ONE, so a +1 is +0. Str can get added to damage if you feel like it, and possibly attack rolls. No bonuses over +3 unless it's truly epic. Con gets added ONCE to HD, so Con +3 would be 5d8+3 on a 5HD creature.
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Example: 5e Harpy has 7HD, AC 11, Challenge 1, a claw attack (+3/2d4+1) and a club (+3/1d4+1). The only notable stats are int -2 and dex +1, which adds to AC so we'll just pretend we're actually adding up ACs.
- 1 HD is probably too low, so 2-3HD is logical.
- AC 11 is a AAC12, or 7 [12] in S&W format.
- Damage added together is 3d4 .
- OSR monsters tend to have multiple claw attacks, so 2 claws at 1d4 each and a club at 1d4.
- A -1 to Int puts them in the "Low" range if that matters.
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Example 2: 5e Annis has 12d10 HD; AC 17; Challenge 6, a bite (+8/3d6+5); 2 claws (+8/3d6+5), and a Crushing Hug that does 9d6+5/round if it grapples. Ability bonuses, subtracting 1, are Str +4, Con +1, Wis +1, Chr +1. The spell abilities (disguise self, fog cloud) are minor.
- Again, 12HD is too high, so we'll drop it to 7HD (right between Challenge and attack bonus).
- AC 17= 1[18].
- Total damage dice are 9d6; annis are combat monsters so halving and rounding up makes sense - call it 2d6 per claw, 1d6 per bite. Even a strength bonus of +4 seems high though, so we'll go with +3 (2d6+3/2d6+3/1d6+3)
- Crushing Hug would do 5d6+3. S&W doesn't have disguise self, change self, or alter self, so just use the 5e spell for its effects. Obscuring mist replaces fog cloud.