Cheapest priced copy (Theros): $0.35 at Card Kingdom, near mint, their price list as of .
Loathsome Catoblepas 
Creature — Beast · 3/3 · Common · Theros · #95
{2}{G}: This creature must be blocked this turn if able.
When this creature dies, target creature an opponent controls gets -3/-3 until end of turn.
Legal inCommanderPioneerModernLegacyVintagePauper
Not legal inBrawlStandard
1 printing · $0.35 to $0.49 at Card Kingdom
| Set | Number | Rarity | Card Kingdom | Foil |
|---|---|---|---|---|
| Theros | 95 | Common | $0.35 | $0.49 |
2 rulings from Wizards of the Coast
- 15 September 2013The activated ability doesn't target any creature or make any specific creature have to block. Activating it more than once in a turn provides no additional benefit.
- 15 September 2013If Loathsome Catoblepas is attacking and its activated ability has resolved, the defending player must assign at least one blocker to it during the declare blockers step if that player controls any creatures that could block it.