As far as I can tell, these haven't been converted under their own or any other name...
Muskrat
I would suggest taking the Enworld Beaver conversion and reducing it 1 size category, since Muskrats should be pretty similar to Tiny Beavers as far as their 3E stats go.
The Enworld Beaver has Str 13 which would be 9 if Tiny-fied. A muskrat is about 4 times bigger than a large rat, so Str 7 would seem better.
I also gave them a rat's mental stats rather than a beaver's, since the Enworld beaver has Wisdom 16 for some weird reason.
That would give stats something like:
Muskrat
Tiny Animal
Hit Dice: 1/2d8 (2 hp)
Initiative: +0
Speed: 10 ft. (2 squares), swim 30 ft.
Armor Class: 12 (+2 size), touch 12, flat-footed 12
Base Attack/Grapple: +0/-10
Attack: Bite +2 melee (1d3-2)
Full Attack: Bite +2 melee (1d3-2)
Space/Reach: 2 1/2 ft./0 ft.
Special Attacks: —
Special Qualities: Cold tolerant, hold breath, low-light vision, scent
Saves: Fort +2, Ref +2, Will +1
Abilities: Str 7, Dex 10, Con 11, Int 2, Wis 12, Cha 2
Skills: Listen +7, Spot +3, Swim +8
Feats: Alertness
Environment: Temperate swamps
Organization: Solitary, pair, family (2-12) or lodge (10–40)
Challenge Rating: 1/8
Treasure: None
Alignment: Always neutral
Advancement: —
Level Adjustment: —
Cold Tolerant (Ex): A muskrat's insulating fat grants it a +2 racial bonus on saving throws made against cold attacks and environmental conditions.
Hold Breath (Ex): A muskrat can hold its breath for a number of rounds equal to 6 x its Constitution score before it risks drowning.
Skills: A muskrat has a +8 racial bonus on any Swim check to perform some special action or avoid a hazard. It can always choose to take 10 on a Swim check, even if distracted or endangered. It can use the run action while swimming, provided it swims in a straight line.