AbilityHeavyweight
As long as there is a Grass Energy attached to Muk, you must pay an additional ColorlessColorless to retreat it.

Aquapolis is a set in the E-Card series. It was released on January 15, 2003. The printed size is 147. This checklist is longer, at 182 cards. Muk is H17/147, in position 167 of 182 on this checklist. Before it is H16 Magneton, and after it is H18 Nidoking. The listed rarity is Rare Holo.
Muk is a Stage 1 Grass Pokémon with 70 HP. In the Pokédex this is Muk (#089), generation 1 (Kanto). The card says it evolves from Grimer. In this catalog that name is Grimer (#088).
Hajime Kusajima illustrated this card. The same name is on 206 other cards in the catalog.
Muk (#089) has 31 other cards in the catalog. Those other cards are in 24 sets. The other printings with a release date run from 1999 to 2025. The earliest other card is Muk 13/62 in Fossil (1999). Both list 70 HP. Both list the rarity Rare Holo. Mitsuhiro Arita illustrated that card, and Hajime Kusajima illustrated this one. Retreat cost there is 2, and here it is 1. 26 other cards of Muk list a different HP.
Heavyweight is listed as Poké-Body. As long as there is a Grass Energy attached to Muk, you must pay an additional ColorlessColorless to retreat it. The attack Burning Sludge lists 10 damage. The cost is Colorless, Colorless. Flip a number of coins equal to the amount of Grass Energy attached to Muk. If any of them are heads, the Defending Pokémon is now Poisoned and Burned. Weakness is Psychic ×2. Retreat cost is 1.
I keep a set binder for Aquapolis, meant to hold every card of that set. This card is one slot in the Aquapolis set binder. A generation binder holds one card of each species. Muk can fill a slot in Generation 1 binder (Kanto). I still need this card.
As long as there is a Grass Energy attached to Muk, you must pay an additional ColorlessColorless to retreat it.
Flip a number of coins equal to the amount of Grass Energy attached to Muk. If any of them are heads, the Defending Pokémon is now Poisoned and Burned.
Aquapolis, in the E-Card series. It was released on January 15, 2003. The printed size is 147.
H17/147. It is entry 167 of 182 in this checklist.
Hajime Kusajima. The catalog lists 206 other cards with that illustrator.
Rare Holo.
Muk #089, generation 1 (Kanto).
Burning Sludge (10).